Process for producing electret, and electrostatic induction conversion device

ABSTRACT

A process is disclosed for producing an electret, which includes a step of thermally treating a composition including a fluorinated polymer and a silane coupling agent, where the fluorinated polymer has an alicyclic structure in its main chain and has a carboxy group or an alkoxycarbonyl group as its terminal group. The silane coupling agent has an amino group; and the temperature for the thermal treatment is from 250° C. to 330° C.

TECHNICAL FIELD

The present invention relates to a process for producing an electret,and an electrostatic induction conversion device.

BACKGROUND ART

Heretofore, an electrostatic induction conversion device such as apower-generating unit or a microphone has been proposed wherein anelectret having an electric charge injected to an insulating material,is used.

As the insulating material for such an electret, it has been common touse a linear fluororesin such as polytetrafluoroethylene.

Recently, it has been proposed to use a polymer having a fluorinatedalicyclic structure in its main chain (e.g. Patent Document 1), as theinsulating material for such an electret. Further, it has also beenproposed to improve the surface voltage by using one having a silanecoupling agent further mixed to a polymer having a fluorinated alicyclicstructure in its main chain and having a carboxy group as its terminalgroup, as the insulating material for an electret (Patent Document 2).Patent Document 2 discloses an Example wherein a polymer having anintrinsic viscosity of 0.23 (corresponding to a weight average molecularweight of 165,000) and a silane coupling agent are mixed and thermallytreated at 200° C.

PRIOR ART DOCUMENTS Patent Documents

Patent Document 1: JP-A-2006-180450

Patent Document 2: WO2008/114489

DISCLOSURE OF INVENTION Technical Problem

However, conventional electrets have a problem such that it is difficultto maintain an injected electric charge stably at a high temperature,and such an electric charge is likely to be discharged with time at ahigh temperature. Such a problem tends to cause reduction of the surfacevoltage of the electret, thus leading to the deterioration of e.g.electrostatic induction performance of an electrostatic inductionconversion device using the electret. Therefore, it has been desired toimprove the charge retention performance, especially the thermalstability, so that the injected electric charge can be stably retained.

An electret is required to have thermal stability in variousapplications. For example, in its application to an electret condensermicrophone (hereinafter referred to as ECM), injected electric chargesare required to be maintained to some extent after a solder reflow step.The maximum temperature of the solder reflow step is about 260° C., andin order to maintain a sufficient performance as ECM, it is importantthat the surface voltage after the step remains to be at least 200 V.The higher the remaining voltage, the higher the performance of ECM.

Whereas, in a case where an electrostatic induction conversion device isused as mounted on a vehicle, particularly around an engine, a long termstability at 125° C. becomes important. In such an application, it isspecifically required that attenuation of the surface voltage of anelectret film is little when the electret is exposed in an environmentof 125° C. for a long time. By conventional electrets, it has beendifficult to satisfy such a requirement.

The present invention has been made in view of the above problem and hasan object to provide a process for producing an electret which has highthermal stability of retained electric charge and which has excellentcharge retention performance, and an electrostatic induction conversiondevice comprising such an electret.

Solution to Problem

In order to accomplish the above object, the present invention providesthe following.

[1] A process for producing an electret, which comprises a step ofthermally treating a composition comprising a fluorinated polymer and asilane coupling agent, wherein the fluorinated polymer has an alicyclicstructure in its main chain and has a carboxy group or an alkoxycarbonylgroup as its terminal group; the silane coupling agent has an aminogroup; and the temperature for the thermal treatment is from 250° C. to330° C.

[2] The process for producing an electret according to [1], wherein thefluorinated polymer has, as the alicyclic structure, a fluorinatedalicyclic structure in its main chain.

[3] The process for producing an electret according to [1] or [2],wherein the fluorinated polymer has, as the alicyclic structure, acyclic structure containing an etheric oxygen atom, in its main chain.

[4] The process for producing an electret according to any one of [1] to[3], wherein the fluorinated polymer has, as the alicyclic structure, afluorinated alicyclic structure containing an etheric oxygen atom, inits main chain.

[5] The process for producing an electret according to any one of [1] to[4], wherein the weight average molecular weight of the fluorinatedpolymer is from 150,000 to 650,000.

[6] The process for producing an electret according to any one of [1] to[5], wherein the silane coupling agent is at least one member selectedfrom the group consisting of γ-aminopropylmethyldiethoxysilane,γ-aminopropylmethyldimethoxysilane,N-(β-aminoethyl)-γ-aminopropylmethyldimethoxysilane,N-(β-aminoethyl)-γ-aminopropylmethyldiethoxysilane,γ-aminopropyltrimethoxysilane, γ-aminopropyltriethoxysilane,N-(β-aminoethyl)-γ-aminopropyltrimethoxysilane,N-(β-aminoethyl)-γ-aminopropyltriethoxysilane andaminophenyltrimethoxysilane.

[7] The process for producing an electret according to any one of [1] to[6], wherein the content of the silane coupling agent is from 0.1 to 20mass %, based on the total amount of the fluorinated polymer and thesilane coupling agent.

[8] The process for producing an electret according to any one of [1] to[7], which includes the following (1) to (4) in the order of (1), (2),(3) and (4):

(1) a step of obtaining a coating fluid having the fluorinated polymerand the silane coupling agent dissolved in a solvent (a coatingfluid-preparation step),

(2) a step of coating a substrate with the coating fluid to form acoating layer comprising the fluorinated polymer and the silane couplingagent (a coating step),

(3) a step of thermally treating the coating layer to obtain a coatingfilm (a thermal treatment step), and

(4) a step of injecting an electric charge to the coating film after thethermal treatment.

[9] The process for producing an electret according to [8], wherein thesolvent is a fluorinated organic solvent.

[10] An electrostatic induction conversion device comprising theelectret obtained by the process as defined in any one of [1] to [9].

ADVANTAGEOUS EFFECTS OF INVENTION

According to the present invention, it is possible to provide anelectret which has high stability with time and thermal stability ofretained electric charge and which has excellent charge retentionperformance, and an electrostatic induction conversion device comprisingsuch an electret.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic diagram illustrating a corona charging equipmentused for injection of electric charge.

FIG. 2 is a diagram showing set positions for measuring points forsurface voltages.

FIG. 3 is a schematic diagram illustrating an equipment used for athermal stability test.

FIG. 4 is shows standardized values of peak areas of infrared absorptionspectra obtained in Test Example 4.

FIG. 5 is a scattering spectrum graph showing a result of measuringsmall-angle X-ray scattering of a coating film.

DESCRIPTION OF EMBODIMENTS

Now, the present invention will be described in further detail.

In this specification, repeating units constituting a polymer may bereferred to simply as “units”.

Further, a compound represented by the formula (1) may be referred toalso as “a compound (1)”. A unit, compound or the like represented byanother formula will be referred to in a similar manner, and forexample, a unit represented by the formula (3-1) may be referred to alsoas “a unit (3-1)”.

The present invention is characterized by comprising a step of thermallytreating a composition for an electret comprising a specific fluorinatedpolymer and a silane coupling agent at a specific temperature.

The present invention preferably includes the following steps in thefollowing order.

(1) A step of obtaining a coating fluid having the specific fluorinatedpolymer and the silane coupling agent dissolved in a solvent (a coatingfluid-preparation step),

(2) a step of coating a substrate with the coating fluid to form acoating layer comprising the specific fluorinated polymer and the silanecoupling agent (a coating step),

(3) a step of thermally treating the coating layer at a specifictemperature to obtain a coating film (a thermal treatment step), and

(4) a step of injecting an electric charge to the coating film after thethermal treatment (an electric charge-injection step).

<Fluorinated Polymer>

The specific fluorinated polymer (hereinafter referred to as the polymer(A)) to be used in the present invention is a fluorinated polymer havingan alicyclic structure in its main chain and having a carboxy group oran alkoxycarbonyl group as its terminal group.

The “alicyclic structure” means a cyclic structure having noaromaticity. Further, “having an alicyclic structure in its main chain”means that among carbon atoms constituting the cyclic structure, atleast one is a carbon atom constituting the main chain of the polymer(A).

The alicyclic structure may be one wherein the cyclic skeleton isconstituted solely by carbon atoms, or may be a heterocyclic structurewhich contains, in addition to carbon atoms, a heteroatom such as anoxygen atom, a nitrogen atom or the like.

For example, a saturated or unsaturated hydrocarbon ring structure whichmay have a substituent, or a heterocyclic structure having some ofcarbon atoms in such a hydrocarbon ring structure substituted by aheteroatom such as oxygen atom, a nitrogen atom or the like, may bementioned.

Among them, an alicyclic structure of a heterocyclic structure having 1or 2 etheric oxygen atoms in its cyclic skeleton, is preferred.

The number of atoms constituting the cyclic skeleton of the alicyclicstructure is preferably from 4 to 7, more preferably 5 or 6. That is,the alicyclic structure is preferably a 4- to 7-membered ring, morepreferably a 5- or 6-membered ring.

Among carbon atoms constituting the alicyclic structure, the carbonatoms constituting the main chain are derived from a polymerizabledouble bond of a monomer used for the polymerization for the polymer(A).

For example, in a case where the fluorinated polymer is a fluorinatedpolymer obtained by polymerization of a cyclic monomer as describedhereinafter, two carbon atoms constituting such a double bond become thecarbon atoms constituting the main chain.

Further, in the case of a fluorinated polymer obtained bycyclopolymerization of a monomer having two polymerizable double bonds,at least two among four carbon atoms constituting the two polymerizabledouble bonds become the carbon atoms constituting the main chain.

The polymer (A) is preferably a fluorinated polymer having a fluorinatedalicyclic structure in its main chain. That is, the alicyclic structurein the main chain is preferably a fluorinated alicyclic structure.

The “fluorinated alicyclic structure” is an “alicyclic structure” havingfluorine atom(s). Further, “having a fluorinated alicyclic structure inits main chain” means that at least one of carbon atoms constituting thefluorinated alicyclic structure is a carbon atom constituting the mainchain of the polymer (A).

Further, in a case where all of the alicyclic structures in the mainchain of the polymer (A) are alicyclic structures other than fluorinatedalicyclic structures, fluorine atoms may be bonded to the main chain notforming a cyclic structure.

The fluorinated alicyclic structure may, for example, be a saturated orunsaturated hydrocarbon ring structure which may have a substituent, orone wherein some or all of hydrogen atoms in e.g. a heterocyclicstructure having some of carbon atoms in such a hydrocarbon ringstructure substituted by a heteroatom such as an oxygen atom, a nitrogenatom or the like, are substituted by fluorine atoms. Among them, afluorinated alicyclic structure of a heterocyclic structure having oneor two etheric oxygen atoms in its cyclic skeleton, is preferred.Further it is preferred that all of hydrogen atoms are substituted byfluorine atoms.

The polymer (A) is preferably the following fluorinated cyclic polymer(I′) or fluorinated cyclic polymer (II′).

Fluorinated cyclic polymer (I′): a polymer having a unit based on acyclic fluorinated monomer.

Fluorinated cyclic polymer (II′): a polymer having a unit formed bycyclopolymerization of a diene-type fluorinated monomer.

Further, the “cyclic polymer” means a polymer having a cyclic structure.

The fluorinated cyclic polymer (I′) has a unit based on the “cyclicfluorinated monomer”.

The “cyclic fluorinated monomer” is a monomer having a polymerizabledouble bond between carbon atoms constituting a fluorinated alicyclicring, or a monomer having a polymerizable double bond between a carbonatom constituting a fluorinated alicyclic ring and a carbon atom ofother than a fluorinated alicyclic ring.

Such a cyclic fluorinated monomer is preferably the following compound(1) or compound (2).

In the above formulae, each of X¹¹, X¹², X¹³, X¹⁴, Y¹¹ and Y¹² which areindependent of one another, is a fluorine atom, a perfluoroalkyl groupor a perfluoroalkoxy group.

The perfluoroalkyl group for X¹¹, X¹², X¹³, X¹⁴, Y¹¹ and Y¹² haspreferably from 1 to 7, more preferably from 1 to 4, carbon atoms. Sucha perfluoroalkyl group is preferably linear or branched, more preferablylinear. Specifically, it may, for example, be a trifluoromethyl group, apentafluoroethyl group or a heptafluoropropyl group, and particularlypreferred is a trifluoromethyl group.

The perfluoroalkoxy group for X¹¹, X¹², X¹³, X¹⁴, Y¹¹ and Y¹² may, forexample, be one having an oxygen atom (—O—) bonded to the aboveperfluoroalkyl group. It may specifically be a trifluoromethoxy group.

X¹¹ is preferably a fluorine atom.

X¹² is preferably a fluorine atom, a trifluoromethyl group or a C₁₋₄perfluoroalkoxy group, more preferably a fluorine atom or atrifluoromethoxy group.

Each of X¹³ and X¹⁴ which are independent of each other, is preferably afluorine atom or a C₁₋₄ perfluoroalkyl group, more preferably a fluorineatom or a trifluoromethyl group.

Each of Y¹¹ and Y¹² which are independent of each other, is preferably afluorine atom, a C₁₋₄ perfluoroalkyl group or a C₁₋₄ perfluoroalkoxygroup, more preferably a fluorine atom or a trifluoromethyl group.

In the compound (1), X¹³ and X¹⁴ may be bonded to each other to form asecond fluorinated alicyclic ring together with the carbon atoms towhich X¹³ and X¹⁴ are bonded.

Such a second fluorinated alicyclic ring is preferably a 4- to6-membered ring.

Such a second fluorinated alicyclic ring is preferably a saturatedalicyclic ring.

Such a second fluorinated alicyclic ring may have an etheric oxygen atom(—O—) in the cyclic skeleton. In such a case, the number of ethericoxygen atoms in the fluorinated alicyclic ring is preferably 1 or 2.

In the compound (2), Y¹¹ and Y¹² may be bonded to each other to form asecond fluorinated alicyclic ring together with the carbon atoms towhich Y¹¹ and Y¹² are bonded.

Such a second fluorinated alicyclic ring is preferably a 4- to6-membered ring.

Such a second fluorinated alicyclic ring is preferably a saturatedalicyclic ring.

Such a second fluorinated alicyclic ring may have an etheric oxygen atom(—O—) in the cyclic skeleton. In such a case, the number of ethericoxygen atoms in the fluorinated alicyclic ring is preferably 1 or 2.

Preferred specific examples of the compound (1) include compounds (1-1)to (1-5).

Preferred specific examples of the compound (2) include compounds (2-1)and (2-2).

The fluorinated cyclic polymer (I′) may be constituted solely by a unitformed by the above cyclic fluorinated monomer, or may be a copolymerhaving such a unit and another unit.

However, in such a fluorinated cyclic polymer (I′), the proportion ofthe unit based on the cyclic fluorinated monomer is preferably at least20 mol %, more preferably at least 40 mol %, or may be 100 mol %, basedon the total of all repeating units constituting the fluorinated cyclicpolymer (I′).

Said another monomer may be one copolymerizable with the above cyclicfluorinated monomer and is not particularly limited. Specifically, theafter-mentioned diene-type fluorinated monomer, a monomer having areactive functional group in the side chain, tetrafluoroethylene,chlorotrifluoroethylene or perfluoro(methyl vinyl ether) may, forexample, be mentioned.

Further, a polymer obtainable by copolymerization of the cyclicfluorinated monomer with the diene-type fluorinated monomer is regardedas the fluorinated cyclic polymer (I′).

The monomer having the above reactive functional group in the sidechain, copolymerizable with the above cyclic fluorinated monomer, may bea fluorinated monomer such as methyl2,2,3,3,4,4-hexafluoro-4-(1,2,2-trifluorovinyloxy)butanoate, methyl2,2,3,3-tetrafluoro-3-(1,1,2,2,3,3-hexafluoro-3-(1,2,2-trifluorovinyloxy)propoxy)propanoate,1,1,2,2-tetrafluoro-2-(1,1,1,2,3,3-hexafluoro-3-(1,2,2-trifluorovinyloxy)propan-2-yloxy)ethanesulfonylfluoride or1,1,2,2-tetrafluoro-2-(1,2,2-trifluorovinyloxy)ethanesulfonyl fluoride,or a hydrocarbon monomer such as hydroxyethyl vinyl ether, hydroxypropylvinyl ether, hydroxybutyl vinyl ether, 2-(2-(vinyloxy)ethoxy)ethanol,methyl acrylate or hydroxyethyl acrylate.

As the fluorinated cyclic polymer (I′), it is preferred to use ahomopolymer of a cyclic fluorinated monomer selected from compounds(1-1), (1-3) and (2-2), or a copolymer of one cyclic fluorinated monomerselected from the three compounds with one member selected fromtetrafluoroethylene, chlorotrifluoroethylene and the after-mentioneddiene-type fluorinated monomer.

It is most preferred to employ a copolymer of compound (1-1) withtetrafluoroethylene, or a copolymer of compound (1-1) with thediene-type fluorinated monomer.

As the diene-type fluorinated monomer to be used here, it is preferredto employ perfluorobutenyl vinyl ether (CF₂═CFOCF₂CF₂CF═CF₂), orperfluoro(4-methylbutenyl) vinyl ether (CF₂═CFOCF(CF₃)CF₂CF═CF₂), and itis most preferred to employ perfluorobutenyl vinyl ether.

The fluorinated cyclic polymer (II′) has a unit formed bycyclopolymerization of “a diene type fluorinated monomer”.

The “diene-type fluorinated monomer” is a monomer having twopolymerizable double bonds and fluorine atoms. Such polymerizable doublebonds are not particularly limited, but are preferably vinyl groups,allyl groups, acryloyl groups or methacryloyl groups.

The diene-type fluorinated monomer is preferably the following compound(3).CF₂═CF-Q-CF═CF₂  (3)

In the formula, Q is a C₁₋₅, preferably C₁₋₃, perfluoroalkylene groupwhich may have a branch and which may have an etheric oxygen atom andwherein some of fluorine atoms may be substituted by halogen atoms otherthan fluorine atoms. Such halogen atoms other than fluorine atoms may,for example, be chlorine atoms or bromine atoms.

Q is preferably a perfluoroalkylene group having an etheric oxygen atom,and in such a case, the etheric oxygen atom in the perfluoroalkylenegroup may be present at one terminal of the group or may be present atboth terminals of the group, or may be present between carbon atoms ofthe group. From the viewpoint of the cyclopolymerizability, it ispreferably present at one terminal of the group.

The following compounds may be mentioned as specific examples of thecompound (3).

CF₂═CFOCF₂CF═CF₂,

CF₂═CFOCF(CF₃)CF═CF₂,

CF₂═CFOCF₂CF₂CF═CF₂,

CF₂═CFOCF₂CF(CF₃)CF═CF₂,

CF₂═CFOCF(CF₃)CF₂CF═CF₂,

CF₂═CFOCFClCF₂CF═CF₂,

CF₂═CFOCCl₂CF₂CF═CF₂,

CF₂═CFOCF₂OCF═CF₂,

CF₂═CFOC(CF₃)₂OCF═CF₂,

CF₂═CFOCF₂CF(OCF₃)CF═CF₂,

CF₂═CFCF₂CF═CF₂,

CF₂═CFCF₂CF₂CF═CF₂,

CF₂═CFCF₂OCF₂CF═CF₂.

As the unit to be formed by cyclopolymerization of the compound (3), thefollowing units (3-1) to (3-4) may be mentioned.

As the unit to be formed by cyclopolymerization of the compound (3),more specifically, the following units (3-a) to (3-k) may be mentioned.In the units (3-a) to (3-k), one of x and y is 0, and the other is 1.

Here, the units (3-a) to (3-k) correspond to the above unit (3-1) whenx=0 and y=1, and they correspond to the above unit (3-2) when x=1 andy=0.

The fluorinated cyclic polymer (II′) may be constituted solely by a unitformed by cyclopolymerization of the above diene-type fluorinatedmonomer, or may be a copolymer having such a unit and another unit.

However, in such a fluorinated cyclic polymer (II′), the proportion ofthe unit formed by cyclopolymerization of the diene-type fluorinatedmonomer is preferably at least 50 mol %, more preferably at least 80 mol%, most preferably 100 mol %, based on the total of all repeating unitsconstituting the fluorinated cyclic polymer (II′).

Said another monomer may be one copolymerizable with the abovediene-type fluorinated monomer and is not particularly limited.Specifically, a cyclic fluorinated monomer such as the above-mentionedcompound (1) or (2), a monomer having the above reactive functionalgroup in the side chain, tetrafluoroethylene, chlorotrifluoroethylene,or perfluoro(methyl vinyl ether) may, for example, be mentioned.

The monomer having the above reactive functional group in the sidechain, copolymerizable with the above diene type fluorinated monomer,may, for example, be a fluorinated monomer such as methyl2,2,3,3,4,4-hexafluoro-4-(1,2,2-trifluorovinyloxy)butanoate, methyl2,2,3,3-tetrafluoro-3-(1,1,2,2,3,3-hexafluoro-3-(1,2,2-trifluorovinyloxy)propoxy)propanoate,1,1,2,2-tetrafluoro-2-(1,1,1,2,3,3-hexafluoro-3-(1,2,2-trifluorovinyloxy)propan-2-yloxy)ethanesulfonylfluoride or1,1,2,2-tetrafluoro-2-(1,2,2-trifluorovinyloxy)ethanesulfonyl fluoride,or a hydrocarbon monomer such as hydroxyethyl vinyl ether, hydroxypropylvinyl ether, hydroxybutyl vinyl ether, 2-(2-(vinyloxy)ethoxy)ethanol,methyl acrylate or hydroxyethyl acrylate.

As the fluorinated cyclic polymer (II′), it is preferred to use ahomopolymer obtainable from one diene-type fluorinated monomer selectedfrom perfluorobutenyl vinyl ether (CF₂═CFOCF₂CF₂CF═CF₂),perfluoro(3-methylbutenyl) vinyl ether (CF₂═CFOCF₂CF(CF₃)CF═CF₂),perfluoro(4-methylbutenyl) vinyl ether (CF₂═CFOCF(CF₃)CF₂CF═CF₂),perfluoro(4-chlorobutenyl) vinyl ether (CF₂═CFOCFClCF₂CF═CF₂),perfluoro(4,4′-dichlorobutenyl) vinyl ether (CF₂═CFOCCl₂CF₂ CF═CF₂) andperfluoro(3-methoxybutenyl) vinyl ether (CF₂═CFOCF₂CF(OCF₃)CF═CF₂), or acopolymer of two or three members selected from the above six diene-typefluorinated monomers or a copolymer of one diene-type fluorinatedmonomer selected from the above six members with tetrafluoroethylene orchlorotrifluoroethylene. It is more preferred to employ a homopolymer ofone diene-type fluorinated monomer selected from the above six members,and it is most preferred to employ a homopolymer of perfluorobutenylvinyl ether or perfluoro(3-methylbutenyl) vinyl ether.

Polymer (A) has a carboxy group or an alkoxycarbonyl group as itsterminal group. The alkoxycarbonyl group may, for example, be amethoxycarbonyl group, an ethoxycarbonyl group, a propoxycarbonyl group,an isopropoxycarbonyl group, a butoxycarbonyl group or atert-butoxycarbonyl group. The terminal group of the polymer (A) ispreferably a carboxy group, a methoxycarbonyl group or an ethoxycarbonylgroup.

The carboxy group or the alkoxycarbonyl group may be present at a mainchain terminal or at a side chain terminal, or at both the main chainterminal and the side chain terminal. From the production efficiency, itis particularly preferably present at a main chain terminal.

The polymer (A) having a carboxy group or an alkoxycarbonyl group at amain chain terminal may be obtained by carrying out polymerization bymeans of a polymerization initiator and decomposing an unstable terminalgroup thereby formed, by e.g. thermal treatment, to form acarbonylfluoride group (—CF═O group) at the terminal, followed by posttreatment.

As the above post treatment, the carbonylfluoride group may behydrolyzed to convert it to a carboxy group. Otherwise, it may bereacted with an alcohol to convert it to an alkoxycarbonyl group.

As the polymerization initiator, one commonly employed may be used, anda polymerization initiator having a peroxide group is particularlypreferred. As a polymerization initiator having a peroxide group, eithera hydrocarbon type polymerization initiator or a fluorinatedpolymerization initiator may be used.

As the hydrocarbon type polymerization initiator, specifically,diisopropyl peroxydicarbonate, diisobutyl peroxydicarbonate, dipropanoicacid peroxide, dibutanoic acid peroxide, benzoic peroxide ordi-tert-butyl peroxide may, for example, be used.

As the fluorinated polymerization initiator, di-perfluoropropanoic acidperoxide, diperfluorobutanoic acid peroxide, perfluorobenzoic peroxideor di-perfluoro tert-butyl peroxide may, for example, be used.

The polymer (A) is preferably amorphous, since it is thereby excellentin the solubility in a solvent and has good compatibility with theafter-described silane coupling agent.

The weight average molecular weight of polymer (A) is preferably atleast 50,000, more preferably at least 150,000, further preferably atleast 200,000, particularly preferably at least 250,000. If the weightaverage molecular weight is less than 50,000, film formation tends to bedifficult. When it is at least 200,000, the heat resistance of the filmwill be improved, and the thermal stability as an electret will beimproved.

On the other hand, if the number average molecular weight is too large,the polymer tends to be hardly soluble in a solvent, thus leading to aproblem such that the film-forming process is restricted. Therefore, theweight average molecular weight of the polymer (A) is preferably at most1,000,000, more preferably at most 850,000, further preferably at most650,000, particularly preferably at most 550,000.

The intrinsic viscosity of the polymer (A) correlates with the molecularweight of the polymer (A). Accordingly, it preferably has an intrinsicviscosity corresponding to the above preferred molecular weight.

A specific preferred intrinsic viscosity value varies depending upon theunits constituting the polymer (A). For example, in a case where thepolymer (A) is a cyclic polymer of CF₂═CFOCF₂CF₂CF═CF₂, the intrinsicviscosity (30° C.) is preferably from 0.1 to 0.9 dl/g, more preferablyfrom 0.2 to 0.8 dl/g, most preferably from 0.3 to 0.6 dl/g.

Such an intrinsic viscosity is a value measured by using e.g.perfluoro(2-butyltetrahydrofuran) as a solvent.

The polymer (A) preferably has a relative dielectric constant of from1.8 to 8.0, more preferably from 1.8 to 5.0, particularly preferablyfrom 1.8 to 3.0, in consideration of the charge retention performance.Such a relative dielectric constant is a value measured at a frequencyof 1 MHz in accordance with ASTM D150.

Further, as the polymer (A), one having a high volume resistivity and ahigh dielectric breakdown voltage is preferred.

The volume resistivity of the polymer (A) is preferably from 10¹⁰ to10²⁰ Ω·cm, more preferably from 10¹⁶ to 10¹⁹ Ω·cm. Such a volumeresistivity is measured in accordance with ASTM D257.

The dielectric breakdown voltage of the polymer (A) is preferably from10 to 25 kV/mm, more preferably from 15 to 22 kV/mm. Such a dielectricbreakdown voltage is measured in accordance with ASTM D149.

As the polymer (A), one having high hydrophobicity is preferred in orderto exclude water which may be adversely influential over the insulatingproperty and to maintain a high insulating property.

As the polymer (A), a commercial product may be used. As a commercialproduct of a fluorinated polymer which has an alicyclic structurecontaining an etheric oxygen atom in its main chain and which has acarboxy group or an alkoxycarbonyl group at a main chain terminal, CYTOP(registered trademark, manufactured by Asahi Glass Company, Limited) maybe mentioned.

<Silane Coupling Agent>

The silane coupling agent to be used in the present invention is asilane coupling agent having an amino group. As such a silane couplingagent having an amino group, the following ones may be exemplified. (Inthe following, the silane coupling agent having an amino group maysimply be referred to as the silane coupling agent.)

A dialkoxysilane such as γ-aminopropylmethyldiethoxysilane,γ-aminopropylmethyldimethoxysilane,N-(β-aminoethyl)-γ-aminopropylmethyldimethoxysilane orN-(β-aminoethyl)-γ-aminopropylmethyldiethoxysilane.

A trialkoxysilane such as γ-aminopropyltrimethoxysilane,γ-aminopropyltriethoxysilane,N-(β-aminoethyl)-γ-aminopropyltrimethoxysilane orN-(β-aminoethyl)-γ-aminopropyltriethoxysilane.

A silane coupling agent having an aromatic amine structure, such as acompound represented by the following formula (s1) or (s2):ArSi(OR²¹)(OR²²)(OR²³)  (s1)ArSiR²⁴(OR²¹)(OR²²)  (s2)[wherein each of R²¹ to R²⁴ which are independent of one another is ahydrogen atom, a C₁₋₂₀ alkyl group or an aryl group, and Ar is a p-, m-or o-aminophenyl group.]

The following ones may be mentioned as specific examples of the compoundrepresented by the formula (s1) or (s2):

aminophenyltrimethoxysilane, aminophenyltriethoxysilane,aminophenyltripropoxysilane, aminophenyltriisopropoxysilane,aminophenylmethyldimethoxysilane, aminophenylmethyldiethoxysilane,aminophenylmethyldipropoxysilane, aminophenylmethyldiisopropoxysilane,aminophenylphenyldimethoxysilane, aminophenylphenyldiethoxysilane,aminophenylphenyldipropoxysilane, aminophenylphenyldiisopropoxysilane,etc.

Any one of the above silane coupling agents may be used alone, or two ormore of them may be used in combination.

Further, it is also preferred to use a partially hydrolyzed condensateof the above silane coupling agent. It is also preferred to use aco-partially hydrolyzed condensate of the above silane coupling agentwith a tetraalkoxysilane such as tetramethoxysilane, tetraethoxysilaneor tetrapropoxysilane.

In consideration of efficient availability, etc., a particularlypreferred silane coupling agent is at least one member selected fromγ-aminopropylmethyldiethoxysilane, γ-aminopropylmethyldimethoxysilane,N-(β-aminoethyl)-γ-aminopropylmethyldimethoxysilane,N-β-aminoethyl)-γ-aminopropylmethyldiethoxysilane,γ-aminopropyltrimethoxysilane, γ-aminopropyltriethoxysilane,N-(β-aminoethyl)-γ-aminopropyltrimethoxysilane,N-(β-aminoethyl)-γ-aminopropyltriethoxysilane andaminophenyltrimethoxysilane.

The content of the silane coupling agent is preferably from 0.1 to 20mass %, more preferably from 0.3 to 10 mass %, most preferably from 0.5to 5 mass %, based on the total amount of the polymer (A) and the silanecoupling agent. Within such a range, it can be uniformly mixed with thepolymer (A), and it is unlikely to undergo phase separation in thesolution.

As the combination of the fluorinated polymer and the silane couplingagent, preferred is a combination of, as the fluorinated polymer, onemember selected from a copolymer of the above compound (1-1) withtetrafluoroethylene or a diene-type fluorinated monomer, and ahomopolymer of the after-mentioned diene-type fluorinated monomer, and,as the silane coupling agent, one member selected fromγ-aminopropylmethyldiethoxysilane, γ-aminopropylmethyldimethoxysilane,N-(β-aminoethyl)-γ-aminopropylmethyldimethoxysilane,N-(β-aminoethyl)-γ-aminopropylmethyldiethoxysilane,γ-aminopropyltrimethoxysilane, γ-aminopropyltriethoxysilane,N-(β-aminoethyl)-γ-aminopropyltrimethoxysilane,N-(β-aminoethyl)-γ-aminopropyltriethoxysilane andaminophenyltrimethoxysilane. Particularly, most preferred is acombination of, as the fluorinated polymer, a homopolymer of theafter-mentioned diene-type fluorinated monomer, and as the silanecoupling agent, one member selected fromγ-aminopropylmethyldiethoxysilane,N-(β-aminoethyl)-γ-aminopropylmethyldimethoxysilane,γ-aminopropyltriethoxysilane, aminophenyltrimethoxysilane,N-(β-aminoethyl)-γ-aminopropyltrimethoxysilane, N-(β-aminoethyl)-γ-aminopropyltriethoxysilane andaminophenyltrimethoxysilane.

As the diene-type fluorinated monomer to be used here, it is preferredto employ perfluorobutenyl vinyl ether (CF₂═CFOCF₂CF₂CF═CF₂) orperfluoro(4-methylbutenyl) vinyl ether (CF₂═CFOCF(CF₃)CF₂CF═CF₂), and itis most preferred to employ perfluorobutenyl vinyl ether.

<Coating Fluid-Preparation Step>

In the coating fluid-preparation step, a composition comprising thepolymer (A) and the silane coupling agent is made into a coating fluidby means of a solvent.

In order to dissolve both of the polymer (A) and the silane couplingagent thereby to obtain a uniform coating fluid, as the solvent, it ispreferred to use a solvent to dissolve the polymer (A) and a solvent todissolve the silane coupling agent in combination.

If a solvent capable of dissolving both of the polymer (A) and thesilane coupling agent is used, it is possible to obtain a uniformcoating fluid by such a solvent alone.

As the solvent to dissolve the polymer (A), a fluorinated organicsolvent may, for example, be used. As such a fluorinated organicsolvent, an aprotic fluorinated solvent is preferred. The aproticfluorinated solvent is a fluorinated solvent having no proton-donatingproperty. As such aprotic fluorinated solvents, the followingfluorinated compounds may be exemplified.

A polyfluoro aromatic compound such as perfluorobenzene,pentafluorobenzene, 1,3-bis(trifluoromethyl)benzene or1,4-bis(trifluoromethyl)benzene; a polyfluorotrialkylamine compound suchas perfluorotributylamine or perfluorotripropylamine; apolyfluorocycloalkane compound such as perfluorodecalin,perfluorocyclohexane or perfluoro(1,3,5-trimethylcyclohexane); apolyfluoro cyclic ether compound such asperfluoro(2-butyltetrahydrofuran); a perfluoropolyether; and apolyfluoroalkane compound such as perfluorohexane, perfluorooctane,perfluorodecane, perfluorododecane, perfluoro(2,7-dimethyloctane),1,1,2-trichloro-1,2,2-trifluoroethane,1,1,1-trichloro-2,2,2-trifluoroethane,1,3-dichloro-1,1,2,2,3-pentafluoropropane,1,1,1,3-tetrachloro-2,2,3,3-tetrafluoropropane,1,1,3,4-tetrachloro-1,2,2,3,4,4-hexafluorobutane,perfluoro(1,2-dimethylhexane), perfluoro(1,3-dimethylhexane),1,1,2,2,3,3,5,5,5-decafluoropentane,1,1,1,2,2,3,3,4,4,5,5,6,6-tridecafluorohexane,1,1,1,2,2,3,3,4,4,5,5,6,6,7,7,8,8-heptadecafluorooctane,1,1,1,2,2,3,3,4,4,5,5,6,6,7,7,8,8,9,9,10,10-henicosafluorodecane,1,1,1,2,2,3,3,4,4-nonafluorohexane,1,1,1,2,2,3,3,4,4,5,5,6,6-tridecafluorooctane,1,1,1,2,2,3,3,4,4,5,5,6,6,7,7,8,8-heptadecafluorodecane,1,1,1,2,3,4,5,5,5-nonafluoro-4-(trifluoromethyl)pentane,1,1,1,2,2,3,5,5,5-nonafluoro-4-(trifluoromethyl)pentane,3,3-dichloro-1,1,1,2,2-pentafluoropropane or1,3-dichloro-1,1,2,2,3-pentafluoropropane.

These aprotic fluorinated solvents may be used alone or in combinationas a mixture.

As an aprotic fluorinated solvent, a hydrofluoroether (HFE) may also bementioned in addition to the above solvents. As HFE, HFE represented bythe formula R^(a)—O—R^(b) (wherein R^(a) is a C₅₋₁₂ linear or branchedpolyfluoroalkyl group which may have an ether bond, and R^(b) is a C₁₋₅linear or branched alkyl group or polyfluoroalkyl group) is preferred.

If the number of carbon atoms in R^(a) is 4 or less, it is difficult todissolve the polymer (A), and if the number of carbon atoms in R^(a)exceeds 13, it is difficult to obtain such a solvent industrially.Therefore, the number of carbon atoms in R^(a) is selected within arange of from 5 to 12. The number of carbon atoms in R^(a) is preferablyfrom 6 to 10, more preferably from 6 to 7, or from 9 to 10.

A polyfluoroalkyl group is a group having at least two of hydrogen atomsin an alkyl group substituted by fluorine atoms and includes aperfluoroalkyl group having all of hydrogen atoms in an alkyl groupsubstituted by fluorine atoms, or a group having at least two ofhydrogen atoms in an alkyl group substituted by fluorine atoms, andhaving at least one of hydrogen atoms in the alkyl group substituted bya halogen atom other than a fluorine atom. As the halogen atom otherthan a fluorine atom, a chlorine atom is preferred.

The polyfluoroalkyl group is preferably a group having at least 60%,more preferably at least 80%, in number of hydrogen atoms of thecorresponding alkyl group substituted by fluorine atoms. A morepreferred polyfluoroalkyl group is a perfluoroalkyl group.

In a case where R^(a) has an ether bond, if the number of ether bonds istoo many, the solubility will be thereby hindered, and accordingly, thenumber of ether bonds in R^(a) is preferably from 1 to 3, morepreferably from 1 to 2.

If the number of carbon atoms in R^(b) is 6 or more, the solubility ofthe fluorinated cyclic structure-containing polymer will besubstantially hindered. A preferred example of R^(b) is a methyl group,an ethyl group, a trifluoroethyl group, a tetrafluoroethyl group, atetrafluoropropyl group or the like.

If the molecular weight of HFE is too high, not only the viscosity ofthe coating fluid will be increased, but also the solubility of thepolymer (A) will be lowered, and accordingly, it is preferably at most1,000.

Further, in order to increase the solubility of the polymer (A), thefluorine content in HFE is preferably from 60 to 80 wt %. As preferredHFE, the following ones may be exemplified.

F(CF₂)₄OCH₃, HCF₂CF₂OCH₂CF₃, HCF₂CF₂CH₂OCH₂CF₃, F(CF₂)₅OCH₃,F(CF₂)₆OCH₃, F(CF₂)₇OCH₃, F(CF₂)₈OCH₃, F(CF₂)₉OCH₃, F(CF₂)₁₀OCH₃,H(CF₂)₆OCH₃, (CF₃)₂CFCF(OCH₃)CF₂CF₃, F(CF₂)₃OCF(CF₃)CF₂OCH₃,F(CF₂)₃OCF(CF₃)CF₂OCF(CF₃)CF₂OCH₃, F(CF₂)₈OCH₂CH₂CH₃, (CF₃)₂CFCF₂CF₂OCH₃and F(CF₂)₂O(CF₂)₄OCH₂CH₃.

Among HFE, (CF₃)₂CFCF(OCH₃)CF₂CF₃ is particularly suitable as thesolvent.

These HFE may be used alone or in combination as a mixture.

As the fluorinated organic solvent to dissolve the polymer (A), it ispreferred to use only an aprotic fluorinated solvent, since it has alarge dissolving power and is a good solvent.

Further, the boiling point of the fluorinated organic solvent todissolve the polymer (A) is preferably from 65 to 220° C. When theboiling point of the fluorinated organic solvent is at least 100° C., auniform film can easily be formed at the time of the coating.

As the solvent to dissolve the silane coupling agent, a proticfluorinated solvent is preferred. A protic fluorinated solvent is afluorinated solvent having a proton-donating property. As such proticfluorinated solvents, the following ones may be exemplified.

A fluorinated alcohol such as trifluoroethanol,2,2,3,3,3-pentafluoro-1-propanol, 2-(perfluorobutyl)ethanol,2-(perfluorohexyl)ethanol, 2-(perfluorooctyl)ethanol,2-(perfluorodecyl)ethanol, 2-(perfluoro-3-methylbutyl)ethanol,2,2,3,3-tetrafluoro-1-propanol, 2,2,3,3,4,4,5,5-octafluoro-1-pentanol,2,2,3,3,4,4,5,5,6,6,7,7-dodecafluoro-1-heptanol,2,2,3,3,4,4,5,5,6,6,7,7,8,8-hexadecafluoro-1-nonanol,1,1,1,3,3,3-hexafluoro-2-propanol or 1,3,3,4,4,4-hexafluoro-2-butanol.

A fluorinated carboxylic acid such as trifluoroacetic acid,perfluoropropanoic acid, perfluorobutanoic acid, perfluoropentanoicacid, perfluorohexanoic acid, perfluoroheptanoic acid, perfluorooctanoicacid, perfluorononanoic acid, perfluorodecanoic acid,1,1,2,2-tetrafluoropropanoic acid, 1,1,2,2,3,3,4,4-octafluoropentanoicacid, 1,1,2,2,3,3,4,4,5,5-dodecafluoroheptanoic acid or1,1,2,2,3,3,4,4,5,5,6,6-hexadecafluorononanoic acid, an amide of such afluorinated carboxylic acid, a fluorinated sulfonic acid such astrifluoromethanesulfonic acid or heptadecafluorooctanesulfonic acid,etc.

These protic fluorinated solvents may be used alone or in combination asa mixture of two or more of them.

The water content in the solvent to be used for the preparation of thecoating fluid should better be small, and it is preferably at most 100mass ppm, more preferably at most 20 mass ppm.

The concentration of the polymer (A) in the coating fluid is preferablyfrom 0.1 to 30 mass %, more preferably from 0.5 to 20 mass %.

The solid content concentration in the coating fluid may be suitably setdepending upon the thickness of the film to be formed. It is usuallyfrom 0.1 to 30 mass %, preferably from 0.5 to 20 mass %.

Here, for the solid content, the coating fluid having the mass measuredis heated under ordinary pressure at 200° C. for one hour to distill thesolvent off, and the mass of the remaining solid content is measured,whereupon the solid content is calculated.

The coating fluid may contain a tetraalkoxysilane such astetramethoxysilane, tetraethoxysilane or tetrapropoxysilane in additionto the polymer (A), the silane coupling agent having an amino group andthe above solvent.

The coating fluid is preferably obtained by preparing a polymer (A)solution having the polymer (A) dissolved in an aprotic fluorinatedsolvent and a silane coupling agent solution having the silane couplingagent dissolved in a protic fluorinated solvent, respectively, and thenmixing the polymer (A) solution and the silane coupling agent solution.

<Coating Step>

In the coating step, a substrate is coated with the coating fluidobtained in the coating fluid-preparation step, to form a coating layercomprising the polymer (A) and the silane coupling agent.

The coating method may, for example, be a roll coater method, a castingmethod, a dipping method, a spin coating method, a casting-on-watermethod, a Langmuir•Blodgett method, a die coating method, an ink jetmethod or a spray coating method. Otherwise, a printing technique suchas a relief printing method, a gravure printing method, a planographicprinting method, a screen printing method or a flexo printing method,may also be used.

As the substrate to be coated with the coating fluid, any substrate maybe used without selecting the material, so long as it is a substratewhich can be earthed at the time of injecting an electric charge to thecoating layer obtained by the coating. A preferred material may, forexample, be an electrically conductive metal such as gold, platinum,copper, aluminum, chromium or nickel.

Further, it is also possible to use a material other than anelectrically conductive metal, for example, an insulating material suchas an inorganic material such as glass, or an organic polymer materialsuch as polyethylene terephthalate, polyimide, polycarbonate or anacrylic resin, so long as it is one having its surface coated with anelectroconductive metal film by a method such as sputtering, vapordeposition or wet coating.

Further, a semiconductor material such as silicon may also be used solong as it is one having similar surface treatment applied or one wherethe resistance value of the semiconductor material itself is low. Theresistance value of the substrate material is preferably at most 0.1Ω·cm, particularly preferably at most 0.01 Ω·cm, by volume resistivity.

The substrate may be a flat plate having a smooth surface or may be onehaving convexoconcave formed. Otherwise, it may have patterning appliedin various shapes. In a case where the above insulating material is usedas the substrate, convexoconcave or a pattern may be formed on theinsulating material itself, or convexoconcave or a pattern may be formedon a metal layer applied on the surface.

The method for forming convexoconcave or a pattern on the substrate isnot particularly limited, and a conventional method may be used. As themethod for forming convexoconcave or a pattern, either a vacuum processor a wet system process may be employed. Specific examples of such amethod include, as the vacuum process, a sputtering method via a maskand a vapor deposition method via a mask; and, as the wet systemprocess, a roll coater method, a casting method, a dipping method, aspin coating method, a casting-on-water method, a Langmuir Blodgettmethod, a die coating method, an ink jet method, a spray coating method,etc. Further, a printing technique such as a relief printing method, agravure printing method, a planographic printing method, a screenprinting method or a flexo printing method may also be used. Further, asa method for forming fine convexoconcave or pattern, a nanoimprintingmethod or a photolithography method may also be used.

The shape and size of the coating layer may suitably be set depending onthe shape and size of the desired electret. The electret is usually usedin the form of a film having a thickness of from 1 to 200 μm. It isparticularly preferred to employ it in the form of a film having athickness of from 10 to 20 μm, from the viewpoint of the characteristicsas an electret and processing efficiency.

In order to make the thickness after the thermal treatment to be from 1to 200 μm, preferably from 10 to 20 μm, the thickness of the coatinglayer may be made to be from 2 to 220 μm, preferably from 12 to 25 μm.

Further, the substrate may be peeled after the electric charge-injectionstep.

<Thermal Treatment Step>

In the thermal treatment step, the above coating layer is thermallytreated at a specific temperature to obtain a coating film (the coatingfilm obtained by the thermal treatment of the coating layer mayhereinafter sometimes be referred to as a coating film).

The thermal treatment preferably comprises a preliminary drying step andthe main drying/baking step.

In the preliminary drying step, the solvent in the coating layer isevaporated as far as possible to preliminarily dry the coating layer. Bythis preliminary drying step, it is possible to prevent foaming of thefilm, surface roughening, non-uniformity, etc. in the subsequent maindrying/baking step. The preliminary drying step is preferably carriedout at a temperature of not higher than the boiling point of thesolvent. Specifically, the temperature is preferably from 50 to 150° C.,more preferably from 80 to 120° C. The time for the preliminary dryingstep is preferably from 0.1 to 5 hours, more preferably from 0.5 to 2hours.

The main drying/baking step is carried out within a range of from 250°C. to 330° C. The temperature (baking temperature) in the maindrying/baking step is preferably within a range of from 260° C. to 300°C., more preferably from 260 to 280° C. By adjusting the bakingtemperature to be the above temperature, it is possible to obtain anelectret having a sufficient surface voltage and being excellent in thethermal stability. The time for the baking step is preferably from 0.5to 5 hours, more preferably from 1 to 2 hours.

The atmosphere for the main drying/baking step may be an inert gasatmosphere or an air atmosphere, but an air atmosphere is preferred forthe formation of amide bonds which will be described hereinafter.Further, the pressure is preferably ordinary pressure.

<Another Layer>

In the present invention, as the case requires, another layer may belaminated on the coating film obtained by thermal treatment of thecoating layer comprising the above polymer (A) and the silane couplingagent. As another layer which may be laminated, a protective layer, alayer composed solely of the polymer (A) or a layer composed of aninorganic substance, may, for example, be mentioned.

Such another layer may be formed on the coating film after the thermaltreatment step, or another layer may be formed at a stage where thepreliminary drying step in the thermal treatment step has been completedand may be baked together with the coating layer comprising the polymer(A) and the silane coupling agent.

<Electric Charge-Injection Step>

As the step for injecting an electric charge to the above coating film,any method may be employed without selecting the means, so long as it isa common method for charging an insulating material. For example, it ispossible to apply e.g. a corona discharge method disclosed in e.g. G. M.Sessler, Electrets Third Edition, pp 20, Chapter 2.2 “Charging andPolarizing Methods” (Laplacian Press, 1998), or an electron beambombardment method, an ion beam bombardment method, a radiation method,a light irradiation method, a contact charging method or a liquidcontact charging method. Especially for the electret of the presentinvention, it is preferred to employ a corona discharge method or anelectron beam bombardment method.

Further, with respect to the temperature condition at the time ofinjecting an electric charge, it is preferred to carry out the injectionat a temperature of at least the glass transition temperature of thepolymer (A) from the viewpoint of the stability of the electric chargeto be maintained after the injection, and it is particularly preferredto carry out the injection under a temperature condition at a level ofthe glass transition temperature+10 to 20° C. Further, with respect tothe voltage to be applied at the time of injecting an electric charge,it is preferred to apply a high voltage so long as it is lower than theinsulation breakdown voltage of the coating film of the polymer (A) andits composition. With the coating film in the present invention, it ispossible to apply a high voltage of from ±6 to ±30 kV, and it isparticularly preferred to apply a voltage of from ±8 to ±15 kV.Particularly, the polymer (A) to be used for the coating film is afluorinated polymer, whereby a negative charge can be maintained morestably than a positive charge, and it is further preferred to apply avoltage of from −8 to −15 kV.

<Electrostatic Induction Conversion Device>

The electret of the present invention is useful for an electrostaticinduction conversion device to covert electric energy to kinetic energy.

Such an electrostatic induction conversion device may, for example, be avibration-type power-generating unit, a microphone, a speaker, anactuator or a sensor. The structure of such an electrostatic inductionconversion device may be the same as a conventional one except that asthe electret, the electret of the present invention is used.

As compared with conventional electrets, the electret obtained by thepresent invention has high stability with time and thermal stability ofretained electric charge and has excellent charge retention performance.

Therefore, the electrostatic induction conversion device of the presentinvention comprising such an electret has such features thatdeterioration of properties is less likely to occur, and dependence ofthe properties on the environment is small.

<Operation Mechanism>

By using the composition comprising the polymer (A) and the silanecoupling agent having an amino group, it is possible not only to improvethe adhesion to a substrate of the coating film made of such acomposition but also to improve the thermal stability of an electriccharge maintained by the electret prepared therefrom.

The reason as to why the thermal stability is improved by incorporatingthe polymer (A) and the silane coupling agent having an amino group, isconsidered to be such that the polymer (A) and the silane coupling agentinduce a nano phase separation to form a nano cluster structure derivedfrom the silane coupling agent, and such a nano cluster structure willfunction as a site to store an electric charge in the electret.

The present inventors have found that the composition comprising thepolymer (A) and the silane coupling agent having an amino group willhave the thermal stability improved by the thermal treatment. It isconsidered that by the thermal treatment, the following reactions arepromoted, whereby the thermal motion of a nano cluster structure derivedfrom the silane coupling agent will be suppressed, and the nano clusterstructure will be stabilized.

(1) A reaction wherein a carboxy group or an alkoxycarbonyl group of thepolymer (A) is reacted with an amino group of the silane coupling agent,whereby the polymer (A) and the silane coupling agent will be bonded.

(2) A reaction wherein silane coupling agents are bonded to each otherby a condensation reaction of alkoxysilyl groups.

The present inventors have conducted a further study, and they havefound that when the temperature for the heat treatment is adjustedwithin the specific temperature range of the present invention, theeffect for thermal stability is high and have confirmed from theinfrared spectrum (hereinafter referred to as the IR spectrum) that sucha specific temperature range corresponds to the above reactions (1) and(2).

That is, the thermal stability was high when the temperature for theheat treatment was within a range of from 250 to 330° C., and it wasparticularly high within a range of from 260 to 300° C.

Whereas, from an analysis of the IR spectrum, it has been found that thecontent (the residual amount) of the amino group is low as thetemperature for the thermal treatment is high. This amino group is agroup to be consumed by the reaction (1). Further, it has been foundthat the content (the residual amount) of an alkoxysilyl group is low asthe temperature for the heat treatment is high. This alkoxysilyl groupis a group to be consumed by the reaction (2). That is, it has beenfound that the reactions (1) and (2) are promoted as the temperature ishigh.

On the other hand, the content of the amide group to be formed by thereaction (1) has been found to be high when the temperature for the heattreatment is within a range of from 250 to 330° C., and is higher withina range of from 260 to 300° C. This indicates that if the temperaturebecomes too high, the formed amide group is likely to be thermallydecomposed.

Further, the present inventors have confirmed a non-uniform structurecorresponding to the above-described nano cluster structure by a smallangle X-ray scattering analysis. And, they have found that as thetemperature for the thermal treatment is high, the non-uniform structuretends to be large. This indicates the following state. That is, as theabove reactions (1) and (2) are more promoted, the nano clusterstructure derived from the silane coupling agent, which functions as asite to store electric charge becomes large. It is considered that as aresult, the thermal stability of the charge-retention performance of theelectret will be improved.

The present inventors have further found that as the molecular weight ofthe polymer (A) is large, the effect for improvement of the thermalstability tends to be large.

It is considered that as the molecular weight is large, the polymer (A)tends to hardly move, and as a result, also the nano cluster structurederived from the silane coupling agent which is present as surrounded bythe polymer (A) tends to hardly undergo thermal motion.

From the foregoing, it is considered that the charge retention propertyof the electret will be improved by a synergistic effect of the largenano cluster structure and suppression of the thermal motion due to anincrease of the weight average molecular weight of the polymer (A).

EXAMPLES

Now, specific cases of the above embodiment will be described asExamples. However, it should be understood that the present invention isby no means restricted to the following Examples.

In the following Examples, the volume resistivity is a value measured inaccordance with ASTM D257.

The dielectric breakdown voltage is a value measured in accordance withASTM D149.

The relative dielectric constant is a value measured in accordance withASTM D150 at a frequency of 1 MHz.

The intrinsic viscosity [η] (30° C.) (unit: dl/g) is a value measured byan Ubbelohde viscometer at 30° C. by usingperfluoro(2-butyltetrahydrofuran) as a solvent.

Of the polymer of perfluorobutenyl vinyl ether, the weight averagemolecular weight is a value calculated from the above intrinsicviscosity in accordance with the following relational formula disclosedin Journal of Chemical Society of Japan, 2001, NO. 12, P. 661.

Calculation formula disclosed in the literature: [η]=1.7×10⁻⁴×Mw^(0.60)where Mw is the weight average molecular weight.

Further, in each of the following Examples, the measurement of the filmthickness was carried out by using optical interfero type film thicknessmeasuring apparatus C10178 manufactured by Hamamatsu Photonics K.K.

Preparation Example 1 Preparation of Polymer Composition Solution M1

(1) Preparation of Polymer Solution

45 g of perfluorobutenyl vinyl ether (CF₂═CFOCF₂CF₂CF═CF₂), 240 g ofdeionized water, 16 g of methanol and 0.2 g of diisopropylperoxydicarbonate powder (((CH₃)₂CHOCOO)₂) as a polymerization initiatorwere put into a pressure resistant glass autoclave having an internalcapacity of 1 L. The interior of the system was flushed three times withnitrogen, and then, suspension polymerization was carried out at 40° C.for 23 hours. As a result, 40 g of polymer A1 was obtained. The infraredabsorption spectrum of this polymer was measured, whereby no absorptionwas observed in the vicinity of 1,660 cm⁻¹ and 1,840 cm⁻¹ attributableto double bonds present in the monomer.

Polymer A1 was thermally treated in air at 250° C. for 8 hours and thenimmersed in water to obtain polymer A2 having a —COOH group as itsterminal group. The IR spectrum of a compression-molded film of such apolymer was measured, whereby a characteristic absorption was observedat 1,775 and 1,810 cm⁻¹ attributable to the —COOH group. Further, theintrinsic viscosity [η] (30° C.) of this polymer was 0.24 dl/g, and theweight average molecular weight of the polymer quantified from such aresult was 177,000. From these results, the product prepared by theabove method was confirmed to be a fluorinated polymer having analicyclic structure in its main chain and at the same time, having acarboxy group or an alkoxycarbonyl group as its terminal group.

The volume resistivity of polymer A2 was >10¹⁷ Ω·cm, the dielectricbreakdown voltage was 19 kV/mm, and the relative dielectric constant was2.1.

With respect to polymer A2, differential scanning calorimetry (DSC) wascarried out, whereby the glass transition temperature (Tg) of polymer A2was 108° C.

In perfluorotributylamine, the above polymer A2 was dissolved at aconcentration of 15 mass % to obtain polymer solution P1.

(2) Incorporation of Silane Coupling Agent

A solution having 10.6 g of perfluorotributylamine added to 84.6 g ofthe above polymer solution P1 was prepared. To this solution, a silanecoupling agent solution (a solution having 0.4 g ofγ-aminopropylmethyldiethoxysilane dissolved in 4.7 g of2-(perfluorohexyl)ethanol) was mixed to obtain a uniform polymercomposition solution M1.

<Preparation of Polymer Composition Solution>

Preparation Example 2 Preparation of Polymer Composition Solution M2

(1) Preparation of Polymer Solution

45 g of perfluorobutenyl vinyl ether (CF₂═CFOCF₂CF₂CF═CF₂), 240 g ofdeionized water, 7 g of methanol and 0.1 g of diisopropylperoxydicarbonate powder (((CH₃)₂CHOCOO)₂) as a polymerization initiatorwere put into a pressure resistant glass autoclave having an internalcapacity of 500 mL. The interior of the system was flushed three timeswith nitrogen, and then, suspension polymerization was carried out at40° C. for 23 hours. As a result, 39 g of polymer B1 was obtained. Theinfrared absorption spectrum of this polymer was measured, whereby noabsorption was observed in the vicinity of 1,660 cm⁻¹ and 1,840 cm⁻¹attributable to double bonds present in the monomer.

Polymer B1 was thermally treated in air at 250° C. for 8 hours and thenimmersed in water to obtain polymer B2 having a —COOH group as itsterminal group. The IR spectrum of a compression-molded film of such apolymer was measured, whereby a characteristic absorption was observedat 1,775 and 1,810 cm⁻¹ attributable to the —COOH group. Further, theintrinsic viscosity [η] (30° C.) of this polymer was 0.32 dl/g, and theweight average molecular weight of the polymer quantified from such aresult was 287,000. From these results, the product prepared by theabove method was confirmed to be a fluorinated polymer having analicyclic structure in its main chain and at the same time, having acarboxy group or an alkoxycarbonyl group as its terminal group.

The volume resistivity of polymer B2 was >10¹⁷ Ω·cm, the dielectricbreakdown voltage was 19 kV/mm, and the relative dielectric constant was2.1.

With respect to polymer B2, differential scanning calorimetry (DSC) wascarried out, whereby the glass transition temperature (Tg) of polymer B2was 108° C.

In perfluorotributylamine, the above polymer B2 was dissolved at aconcentration of 11 mass % to obtain polymer solution P2.

(2) Incorporation of Silane Coupling Agent

To 76.3 g of the above polymer solution P2, a silane coupling agentsolution (a solution having 0.3 g of γ-aminopropylmethyldiethoxysilanedissolved in 4.4 g of 2-(perfluorohexyl)ethanol) was mixed to obtain auniform polymer composition solution M2.

Preparation Example 3 Preparation of Polymer Composition Solution M3

(1) Preparation of Polymer Solution

150 g of perfluorobutenyl vinyl ether (CF₂═CFOCF₂CF₂CF═CF₂), 650 g ofdeionized water and 0.3 g of diisopropyl peroxydicarbonate powder(((CH₃)₂CHOCOO)₂) as a polymerization initiator were put into a pressureresistant glass autoclave having an internal capacity of 2 L. Theinterior of the system was flushed three times with nitrogen, and then,suspension polymerization was carried out at 40° C. for 23 hours. As aresult, 123 g of polymer C1 was obtained. The IR spectrum of thispolymer was measured, whereby no absorption was observed in the vicinityof 1,660 cm⁻¹ and 1,840 cm⁻¹ attributable to double bonds present in themonomer.

Polymer B1 was thermally treated in air at 250° C. for 8 hours and thenimmersed in water to obtain polymer C2 having a —COOH group as itsterminal group. The infrared absorption spectrum of a compression-moldedfilm of such a polymer was measured, whereby a characteristic absorptionwas observed at 1,775 and 1,810 cm⁻¹ attributable to the —COOH group.Further, the intrinsic viscosity [η] (30° C.) of this polymer was 0.45dl/g, and the weight average molecular weight of the polymer quantifiedfrom such a result was 506,000. From these results, the product preparedby the above method was confirmed to be a fluorinated polymer having analicyclic structure in its main chain and at the same time, having acarboxy group or an alkoxycarbonyl group as its terminal group.

The volume resistivity of polymer C2 was >10¹⁷ Ω·cm, the dielectricbreakdown voltage was 19 kV/mm, and the relative dielectric constant was2.1.

With respect to polymer C2, differential scanning calorimetry (DSC) wascarried out, whereby the glass transition temperature (Tg) of polymer C2was 108° C.

In perfluorotributylamine, the above polymer C2 was dissolved at aconcentration of 9 mass % to obtain polymer solution P3.

(2) Incorporation of Silane Coupling Agent

To 44.4 g of the above polymer solution P3, a silane coupling agentsolution (a solution having 0.1 g of γ-aminopropylmethyldiethoxysilanedissolved in 2.4 g of 2-(perfluorohexyl)ethanol) was mixed to obtain auniform polymer composition solution M3.

Preparation Example 4 Preparation of Polymer Composition Solution M4

(1) Preparation of Polymer Solution

Polymer (A1) obtained in Preparation Example 1 was thermally treated inair at 250° C. for 8 hours and then immersed in methanol to obtainpolymer A3 having a —COOCH₃ group as its terminal group. The IR spectrumof a compression-molded film of the polymer was measured, whereby acharacteristic absorption was observed at 1,795 cm⁻¹ attributable to the—COOCH₃ group. Further, the intrinsic viscosity [η] (30° C.) of thispolymer is 0.24 dl/g, and the weight average molecular weight of thepolymer quantified from this result was 177,000. From these results, theproduct prepared by the above method was confirmed to be a fluorinatedpolymer having an alicyclic structure in its main chain and at the sametime, having a carbonyl group or an alkoxycarbonyl group as its terminalgroup.

The volume resistivity of polymer A3 was >10¹⁷ Ω·cm, the dielectricbreakdown voltage was 19 kV/mm, and the relative dielectric constant was2.1.

With respect to polymer A3, differential scanning calorimetry (DSC) wascarried out, whereby the glass transition temperature (Tg) of polymer A3was 108° C.

In perfluorotributylamine, the above polymer A3 was dissolved at aconcentration of 9 mass % to obtain polymer solution P4.

(2) Incorporation of Silane Coupling Agent

To 25.1 g of the above polymer solution P4, a silane coupling agentsolution (a solution having 0.07 g of γ-aminopropylmethyldiethoxysilanedissolved in 1.2 g of 2-(perfluorohexyl)ethanol) was mixed to obtain auniform polymer composition solution M4.

Preparation Example 5 Preparation of Polymer Composition Solution M5

(1) Preparation of Polymer Solution

In accordance with the procedure disclosed in Example 2 in JapanesePatent No. 3,053,657, perfluoro(butenyl vinyl ether) andperfluoro(2,2-dimethyl-1,3-dioxol) (hereinafter referred to as PDD) werepolymerized to obtain polymer D1.

The IR spectrum of polymer D1 was measured, and from the absorbance ofabsorption at 1,930 cm⁻¹, the repeating unit based on PDD (PDD content)contained in the polymer D1 was determined to be 52 mol %.

Such polymer D1 was thermally treated in air at 330° C. for 5 hours andthen immersed in water to obtain polymer D2.

With respect to such polymer D2, differential scanning calorimetry (DSC)was carried out, whereby the glass transition temperature (Tg) ofpolymer D2 was 149° C.

Further, a compression-molded film of the polymer D2 was prepared, andthe IR spectrum of such a molded film was measured, whereby thecharacteristic absorption was observed at 1,775 cm⁻¹ and 1,810 cm⁻¹attributable to the —COOH group.

Further, the intrinsic viscosity [η] (30° C.) of polymer D2 was 0.36dl/g, the volume resistivity was >10¹⁷ Ω·cm, and the relative dielectricconstant was 2.0.

Then, in perfluorotributylamine, the above polymer D2 was dissolved at aconcentration of 11 mass % to obtain polymer solution P5.

(2) Incorporation of Silane Coupling Agent

A solution having 14 g of perfluorotributylamine added to 76.9 g of theabove polymer solution P5 was prepared. To such a solution, a silanecoupling agent solution (a solution having 0.4 g ofγ-aminopropylmethyldiethoxysilane dissolved in 4.4 g of2-(perfluorohexyl)ethanol) was mixed to obtain a uniform polymercomposition solution M5.

Preparation Example 6 Preparation of Polymer Composition Solution M6

(1) Preparation of Polymer Solution

In perfluorotributylamine, commercially available fluorinated polymerTeflon-AF1600 (manufactured by Du Pont Kabushiki Kaisha) was dissolvedat a concentration of 8 mass % to obtain polymer solution P6.

With respect to this Teflon-AF1600 (manufactured by Du Pont KabushikiKaisha), the infrared absorption spectrum of a compression-molded filmof such a polymer was measured, whereby the characteristic absorptionwas observed at 1,810 cm⁻¹ attributable to the —COOH group. Further, theintrinsic viscosity [η](30° C.) of the polymer was 1.05 dl/g, the volumeresistivity was >10¹⁷ Ω·cm, and the relative dielectric constant was1.9. Further, with respect to such a polymer, differential scanningcalorimetry (DSC) was carried out, whereby the glass transitiontemperature (Tg) of the polymer was 164° C.

(2) Incorporation of Silane Coupling Agent

To 53 g of the above polymer solution P6, a silane coupling agentsolution (a solution having 0.13 g of γ-aminopropylmethyldiethoxysilanedissolved in 2.6 g of 2-(perfluorohexyl)ethanol) was mixed to obtain auniform polymer composition solution M6.

Preparation Example 7 Preparation of Polymer Composition Solution M7

(1) Preparation of Polymer Solution

15 g of CF₂═CFCF₂CF(CF₃)OCF═CF₂, 80 g of deionized water, 2.4 g ofmethanol and 38 g of perfluorobenzoyl peroxide as a polymerizationinitiator were put into a stainless steel autoclave having an internalcapacity of 200 mL. Such an autoclave was flushed with nitrogen and thenheated until the internal temperature of the autoclave became 70° C.,followed by polymerization for 20 hours. The obtained polymer was washedwith deionized water and methanol and then dried at 200° C. for onehour. As a result, 12 g of polymer E1 was obtained. The IR spectrum ofthis polymer E1 was measured, whereby no absorption was observed at1,660 cm⁻¹ or 1,840 cm⁻¹ attributable to the double bond present in themonomer.

The intrinsic viscosity (30° C.) of the polymer E1 was measured, andfound to be 0.31 dl/g.

Then, the polymer E1 was thermally treated in air at 250° C. for 8 hoursand then immersed in water to obtain polymer E2 having a —COOH group asits terminal group. The IR spectrum of a compression-molded film of thepolymer was measured, whereby the characteristic absorption was observedat 1,775 cm⁻¹ and 1,810 cm⁻¹ attributable to the —COOH group.

The volume resistivity of the polymer E2 was >10¹⁷ Ω·cm, and therelative dielectric constant was 2.0. Further, the glass transitiontemperature measured by differential scanning calorimetry (DSC) was 124°C.

In perfluorotributylamine, the above polymer E2 was dissolved at aconcentration of 15 mass % to obtain polymer solution P7.

(2) Incorporation of Silane Coupling Agent

A solution having 4.6 g of perfluorotributylamine added to 24 g of theabove polymer solution P7 was prepared. To such a solution, a silanecoupling agent solution (a solution having 0.1 g ofγ-aminopropylmethyldiethoxysilane dissolved in 1.3 g of2-(perfluorohexyl)ethanol) was mixed to obtain a uniform polymercomposition solution M7.

Preparation Example 8 Preparation of Polymer Composition Solution M8

A solution having 6.3 g of perfluorotributylamine added to 69.9 g of thepolymer solution P1 in the above Preparation Example 1, was prepared. Tosuch a solution, a silane coupling agent solution (a solution having 0.3g of γ-aminopropylmethyldiethoxysilane dissolved in 3.5 g of2-(perfluorohexyl)ethanol) was mixed to obtain a uniform polymercomposition solution M8.

Preparation Example 9 Preparation of Polymer Composition Solution M9

A solution having 6.8 g of perfluorotributylamine added to 69.3 g of thepolymer solution P1 in the above Preparation Example 1, was prepared. Tosuch a solution, a silane coupling agent solution (a solution having 0.4g of N-(β-aminoethyl)-γ-aminopropyltriethoxysilane dissolved in 3.5 g of2-(perfluorohexyl)ethanol) was mixed to obtain a uniform polymercomposition solution M9.

Preparation Example 10 Preparation of Polymer Composition Solution M10

To 44.9 g of the polymer solution P1 in the above Preparation Example 1,a silane coupling agent solution (a solution having 0.2 g ofN-(β-aminoethyl)-γ-aminopropylmethyldimethoxysilane dissolved in 2.5 gof 2-(perfluorohexyl)ethanol) was mixed to obtain a uniform polymercomposition solution M10.

Preparation Example 11 Preparation of Polymer Composition Solution M11

To 38.2 g of the polymer solution P1 in the above Preparation Example 1,a silane coupling agent solution (a solution having 0.2 g ofm-aminophenyltrimethoxysilane dissolved in 5.0 g of2-(perfluorohexyl)ethanol) was mixed to obtain a uniform polymercomposition solution M11.

Preparation Example 12 Preparation of Polymer Composition Solution M12

To 76.7 g of the polymer solution P2 in the above Preparation Example 2,a silane coupling agent solution (a solution having 0.3 g ofN-(β-aminoethyl)-γ-aminopropylmethyldimethoxysilane dissolved in 4.3 gof 2-(perfluorohexyl)ethanol) was mixed to obtain a uniform polymercomposition solution M12.

Preparation Example 13 Preparation of Polymer Composition Solution M13

To 44.5 g of the polymer solution P2 in the above Preparation Example 2,a silane coupling agent solution (a solution having 0.2 g ofm-aminophenyltrimethoxysilane dissolved in 6.5 g of2-(perfluorohexyl)ethanol) was mixed to obtain a uniform polymercomposition solution M13.

Test Example 1 Example 1

(1) Preparation of Electret

A copper substrate (3 cm square, thickness: 300 μm) was coated with thepolymer composition solution M1 by a spin coating method, followed bypretreatment at 100° C. for one hour, and then, thermal treatment wascarried out at a thermal treatment temperature of 300° C. for one hourto form a coating film having a thickness of 15 μm.

To this coating film, injection of electric charge was carried out bymeans of a corona charging equipment of which a schematic diagram isshown in FIG. 1, to obtain an electret in Example 1.

This corona charging equipment is designed so that by using the abovecopper substrate (hereinafter referred to as “the copper substrate 10”)having the coating film 11 (hereinafter referred to as “the coating film11”) as an electrode, a high voltage can be applied between a coronaneedle 14 and the copper substrate 10 by a DC high voltage power source12 (HAR-20R5, manufactured by Matsusada Precision Inc.). Further, it isdesigned that to a grid 16, a grid voltage can be applied from a gridpower source 18. It is thereby designed that negative ions dischargedfrom the corona needle 14 are homogenized by the grid 16 and thenshowered down on the coating film 11, whereby electric charge isinjected.

Further, in order to stabilize the electric charge injected to thecoating film 11, it is designed that by a hotplate 19, the coating film11 can be heated to a temperature of at least the glass transitiontemperature during the electric charge-injection step. Here, 17 is anammeter.

In Example 1, the heating temperature of the coating film 11 by thehotplate 19 was adjusted to 120° C. which is higher by 12° C. than theglass transition temperature (Tg: 108° C. in the case of polymer A2) ofthe polymer (polymer A2) used.

And, a high voltage of −8 kV was applied for three minutes between thecorona needle 14 and the copper substrate 10 in the atmospheric airatmosphere. Further, during the period, the grid voltage was set to be−1,100 V.

(2) Measurement of Surface Voltage

With respect to the electret thus prepared, as intended for a solderreflow step required in an application to ECM, the following respectivesurface voltages were measured, and the surface voltage residual ratiowas obtained. The results are shown in Table 1.

Here, the value of each surface voltage in Table 1 is an average valueobtained by measuring surface voltages at 9 measuring points (set in alattice arrangement for every 3 mm from the center of the film, as shownin FIG. 2) of each electret by means of a surface voltmeter (model 279,manufactured by Monroe Electronics Inc.) (the same applies in Table 2 etseq.).

(Initial Surface Voltage)

The surface voltage at the time when an electret immediately afterinjecting electric charge by corona charging was returned to ordinarytemperature (25° C., the same applies hereinafter).

(Surface Voltage Before Heating at 260° C.)

The surface voltage at the time when after storing the electret afterthe measurement of the initial surface voltage for 25 hours under acondition of 20° C. under a relative humidity of 60%, the electret wasreturned to ordinary temperature.

(Surface Voltage after Heating at 260° C.)

The surface voltage at the time when after exerting a thermal history of10 minutes in an oven adjusted to 260° C., to the electret aftermeasuring the surface voltage before heating at 260° C., the electretwas returned to ordinary temperature.

(Surface Voltage Residual Ratio)

The ratio of the surface voltage after heating at 260° C. to the surfacevoltage before heating at 260° C.

Examples 2 to 7 and Comparative Examples 1 to 5

(1) Preparation of Electrets

An electret in each Example or Comparative Example was prepared in thesame manner as in Example 1 except that the polymer composition solutionwas the polymer composition solution in Tables 1 and 2, and thetemperature for the thermal treatment was adjusted to the value inTables 1 and 2.

With respect to each electret thus prepared, the initial surfacevoltage, the surface voltage before heating at 260° C. and the surfacevoltage after heating at 260° C. were measured, and the surface voltageresidual ratio was obtained, in the same manner as in Example 1. Theresults are shown in Tables 1 and 2.

TABLE 1 Comp. Comp. Comp. Ex. 1 Ex. 2 Ex. 3 Ex. 4 Ex. 5 Ex. 1 Ex. 2 Ex.3 Polymer composition M1 M1 M1 M1 M1 M1 M1 M¹ solution Temperature forthermal 300 280 260 250 330 240 230 200 treatment (° C.) Grid voltage(V) −1,100 −1,100 −1,100 −1,100 −1,100 −1,100 −1,100 −1,100 Initialsurface voltage (V) −1,175 −1,188 −1,186 −1,175 −1,174 −1,184 −1,184−1,177 Surface voltage before −1,168 −1,182 −1,179 −1,175 −1,163 −1,176−1,176 −1,170 heating at 260° C. (V) Surface voltage after −497 −392−359 −231 −338 −152 −128 −50 heating at 260° C. (V) Surface voltage 4333 30 20 29 13 11 4 residual ratio (%)

TABLE 2 Comp. Comp. Ex. 6 Ex. 7 Ex. 4 Ex. 5 Polymer composition solutionM2 M3 M2 M3 Temperature for thermal 280 280 200 200 treatment (° C.)Grid voltage (V) −1,100 −1,100 −1,100 −1,100 Initial surface voltage (V)−1,183 −1,105 −1,186 −1,056 Surface voltage −1,172 −1,097 −1,159 −1,051before heating at 260° C. (V) Surface voltage −544 −520 −61 −76 afterheating at 260° C. (V) Surface voltage 46 47 5 7 residual ratio (%)

When the surface voltage residual ratios shown in Table 1 are compared,it is evident that the surface voltage residual ratio in each ofExamples wherein the temperature for the thermal treatment was from 250to 330° C., exceeded the surface voltage residual ratios in ComparativeExamples wherein the temperature for the thermal treatment was less than250° C. The surface voltage residual ratio was particularly high whenthe temperature for the thermal treatment was from 260 to 300° C.

Further, when the surface voltage residual ratios shown in Table 2 arecompared between Examples wherein the same polymer composition solutionwas used, the surface voltage residual ratio in each of Examples whereinthe temperature for the thermal treatment was 280° C., exceeded thesurface voltage residual ratios in Comparative Examples wherein thetemperature for the thermal treatment was 200° C.

Further, in each Example, the surface voltage (absolute value) afterheating at 260° C. exceeded 200 V as the value required for ECM.

Test Example 2 Examples 8 to 19 and Comparative Examples 11 to 15

(1) Preparation of Electrets

An electret in each Example or Comparative Example was prepared in thesame manner as in Example 1 except that the polymer composition solutionwas the polymer composition solution in Tables 3 to 5, the temperaturefor the thermal treatment and the grid voltage were adjusted to thevalues in Tables 3 to 5, and the temperature for heating the coatingfilm in the charge-injection step was adjusted to the followingtemperature which is higher by 12° C. than the glass transitiontemperature of the polymer used.

Examples 8 to 15, 18 and 19, and Comparative Examples 6 to 11, 14 and 15

-   -   Glass transition temperature of the polymer used: 108° C.    -   Temperature for heating the coating film: 120° C.

Example 16 and Comparative Example 12

-   -   Glass transition temperature of the polymer used: 149° C.    -   Temperature for heating the coating film: 161° C.

Example 17 and Comparative Example 13

-   -   Glass transition temperature of the polymer used: 124° C.    -   Temperature for heating the coating film: 136° C.

(2) Measurement of Surface Voltages

With respect to each electret thus prepared, as intended for anelectrostatic induction conversion device for vehicles, the followingrespective surface voltages were measured, and the surface voltageresidual ratio was obtained in the same manner as in Example 1. Theresults are shown in Tables 3 to 5.

(Initial Surface Voltage)

The surface voltage at the time when an electret immediately afterinjecting electric charge by corona charging was returned to ordinarytemperature.

(Surface Voltage Before Heating at 125° C.)

The surface voltage at the time when after storing the electret afterthe measurement of the initial surface voltage for a storage time asshown in Tables 3 to 5 under a condition of 20° C. under a relativehumidity of 60%, the electret was returned to ordinary temperature.

(Surface Voltage after Heating at 125° C.)

The surface voltage at the time when after exerting a thermal history of100 hours in an oven adjusted to 125° C., to the electret aftermeasuring the surface voltage before heating at 125° C., the electretwas returned to ordinary temperature.

(Surface Voltage Residual Ratio)

The ratio of the surface voltage after heating at 125° C. to the surfacevoltage before heating at 125° C.

TABLE 3 Comp. Comp. Comp. Ex. 8 Ex. 9 Ex. 10 Ex. 11 Ex. 12 Ex. 6 Ex. 7Ex. 8 Polymer composition M1 M1 M1 M1 M1 M1 M1 M1 solution Temperaturefor thermal 300 280 260 250 330 240 230 200 treatment (° C.) Gridvoltage (V) −1,100 −1,100 −1,100 −1,100 −1,100 −1,100 −1,100 −1,100Initial surface voltage (V) −1,176 −1,182 −1,186 −1,160 −1,180 −1,183−1,180 −1,178 Storage time (hr) 120 120 120 120 70 120 120 120 Surfacevoltage before −1,165 −1,166 −1,177 −1,149 −1,168 −1,169 −1,168 −1,167heating at 125° C. (V) Surface voltage after −1,012 −983 −944 −870 −927−835 −803 −664 heating at 125° C. (V) Surface voltage 87 84 80 76 79 7169 57 residual ratio (%)

TABLE 4 Ex. 13 Ex. 14 Ex. 15 Ex. 16 Ex. 17 Ex. 18 Ex. 19 Polymercomposition M2 M3 M4 M5 M7 M8 M9 solution Temperature for thermal 280280 280 280 280 280 280 treatment (° C.) Grid voltage (V) −600 −600 −600−600 −600 −600 −600 Initial surface voltage (V) −1,282 −1,278 −1,177−1,217 −1,119 −1,482 −1,437 Storage time (hr) 350 650 210 350 500 430620 Surface voltage before −1,258 −1,255 −1,163 −1,145 −1,092 −1,375−1,329 heating at 125° C. (V) Surface voltage after −1,117 −1,160 −726−1,087 −1,017 −969 −1,073 heating at 125° C. (V) Surface voltage 89 9262 95 93 70 81 residual ratio (%)

TABLE 5 Comp. Comp. Comp. Comp. Comp. Comp. Comp. Ex. 9 Ex. 10 Ex. 11Ex. 12 Ex. 13 Ex. 14 Ex. 15 Polymer composition M2 M3 M4 M5 M7 M8 M9solution Temperature for thermal 200 200 200 200 200 200 200 treatment(° C.) Grid voltage (V) −600 −600 −600 −600 −600 −600 −600 Initialsurface voltage (V) −1,324 −1,401 −1,178 −1,179 −1,039 −1,346 −1,345Storage time (hr) 350 650 210 350 570 480 650 Surface voltage before−1,301 −1,267 −1,162 −1,062 −996 −1,278 −1,328 heating at 125° C. (V)Surface voltage after −828 −791 −438 −889 −806 −728 −668 heating at 125°C. (V) Surface voltage 64 62 38 84 81 57 50 residual ratio (%)

When the surface voltage residual ratios shown in Table 3 are compared,it is evident that the surface voltage residual ratio in each ofExamples wherein the temperature for the thermal treatment was from 250to 330° C., exceeded the surface voltage residual ratios in ComparativeExamples wherein the temperature for the thermal treatment was less than250° C. The surface voltage residual ratio was particularly high whenthe temperature for the thermal treatment was from 260 to 300° C.

Further, when the surface voltage residual ratios shown in Tables 4 and5 are compared between Examples wherein the same polymer compositionsolution was used, it is evident that the surface voltage residual ratioin each of Examples wherein the temperature for the thermal treatmentwas from 260 to 280° C., exceeded the surface voltage residual ratios inComparative Examples wherein the temperature for the thermal treatmentwas 200° C.

Test Example 3 Examples 20 to 32 and Comparative Examples 16 to 26

(1) Preparation of Electrets

An electret in each Example or Comparative Example was prepared in thesame manner as in Example 1 except that the polymer composition solutionwas the polymer composition solution in Tables 6 to 8, the temperaturefor the thermal treatment and the grid voltage were adjusted to thevalues in Tables 6 to 8, and the temperature for heating the coatingfilm in the electric charge injection step was adjusted to the followingtemperature which is higher by 12° C. than the glass transitiontemperature of the polymer used.

(Examples 20 to 27, 31 and 32, and Comparative Examples 16 to 21, 25 and26)

-   -   Glass transition temperature of the polymer used: 108° C.    -   Temperature for heating the coating film: 120° C.

(Example 28 and Comparative Example 22)

-   -   Glass transition temperature of the polymer used: 149° C.    -   Temperature for heating the coating film: 161° C.

(Example 29 and Comparative Example 23)

-   -   Glass transition temperature of the polymer used: 164° C.    -   Temperature for heating the coating film: 176° C.

(Example 30 and Comparative Example 24)

-   -   Glass transition temperature of the polymer used: 124° C.    -   Temperature for heating the coating film: 136° C.

(2) Measurement of Surface Voltage

With respect to each electret thus prepared, the following respectivesurface voltages were measured in the same manner as in Example 1. Theresults are shown in Tables 6 to 8.

(Initial Surface Voltage)

The surface voltage at the time when an electret immediately afterinjecting electric charge by corona charging was returned to ordinarytemperature.

(Surface Voltage Before TSD Test)

The surface voltage at the time when after storing the electret aftermeasuring the initial surface voltage for a storage time as shown inTables 6 to 8 under a condition of 20° C. under a relative humidity of60%, the electret was returned to ordinary temperature.

(3) TSD Test

With respect to the electrets (hereinafter referred to as “electret 21”)after measuring the surface voltages before TSD test, a TSD test wascarried out by the following procedure by using an equipment, of which aschematic diagram is shown in FIG. 3.

Firstly, as shown in FIG. 3, a counter electrode 20 was disposed to facean electret 21 on a copper substrate 10 (the same as the coppersubstrate 10 in FIG. 1).

Then, the temperature at the portion shown by dashed lines in FIG. 3 wasraised at a constant rate (1° C./min) by heating by means of a heater,and the amount of electric charge discharged from each electret 21 wasmeasured as a current value i flowing from the counter electrode 20 byammeter 22 (a fine ammeter (model 6517A manufactured by Keithley)), andthe discharge initiation temperature and the discharge peak temperaturewere obtained. The results are shown in Tables 6 to 8.

Here, the discharge peak temperature represents a temperature at whichthe current value detected at the time of the discharge becomes maximum,and the discharge initiation temperature represents a temperature at thetime when the current value obtained by the following formula (thecurrent value at the initiation of the discharge) was detected by theammeter 22.Current value at the initiation of the discharge={(current value at thedischarge peak temperature)−(current value before thedischarge)}×0.1+(current value before the discharge)

The TSD test is a test by a method so-called a Thermal StimulatedDischarge method (hereinafter referred to as a TSD method). In thismethod, a capacitor will be formed by the electret 21 and the counterelectrode 20. Accordingly, when the electret 21 is heated, the electriccharge trapped in the film tends to be unstable, and if electric chargein the vicinity of the surface diminishes by e.g. diffusion, theelectric charge stored in the counter electrode 20 will also decrease.Thus, by measuring the electric current value flowing from the counterelectrode 20, the thermal stability of each electret can be evaluated.

In the test by the TSD method, both of the discharge peak temperatureand the discharge initiation temperature are important, but thedischarge initiation temperature is particularly important. It is saidthat the higher these temperatures, the higher the thermal stability ofthe electret.

TABLE 6 Comp. Comp. Comp. Ex. 20 Ex. 21 Ex. 22 Ex. 23 Ex. 24 Ex. 16 Ex.17 Ex. 18 Polymer composition M1 M1 M1 M1 M1 M1 M1 M1 solutionTemperature for thermal 300 280 260 250 330 240 230 200 treatment (° C.)Grid voltage (V) −600 −600 −600 −600 −600 −600 −600 −600 Initial surface−1,221 −1,290 −1,229 −1,212 −1,169 −1,184 −1,468 −1,209 voltage (V)Storage time (hr) 350 70 90 120 24 95 190 140 Surface voltage before−1,208 −1,262 −1,124 −1,204 −1,159 −1,204 −1,434 −973 TSD heating (V)Discharge peak 176 223 215 204 166 202 179 183 temperature (° C.)Discharge initiation 162 153 159 146 159 145 134 135 temperature (° C.)

TABLE 7 Ex. 25 Ex. 26 Ex. 27 Ex. 28 Ex. 29 Ex. 30 Ex. 31 Ex. 32 Polymercomposition M2 M3 M4 M5 M6 M7 M8 M9 solution Temperature for thermal 280280 280 280 280 280 280 280 treatment (° C.) Grid voltage (V) −600 −600−600 −600 −600 −600 −600 −600 Initial surface voltage (V) −1,282 −1,278−1,177 −1,031 −818 −1,108 −971 −1,115 Storage time (hr) 350 650 2101,000 700 1,120 360 190 Surface voltage before −982 −1,204 −1,089 −916−549 −1,100 −705 −1,080 TSD heating (V) Discharge peak 251 238 205 247242 239 238 212 temperature (° C.) Discharge initiation 178 172 144 175184 168 161 162 temperature (° C.)

TABLE 8 Comp. Comp. Comp. Comp. Comp. Comp. Comp. Comp. Ex. 19 Ex. 20Ex. 21 Ex. 22 Ex. 23 Ex. 24 Ex. 25 Ex. 26 Polymer composition M2 M3 M4M5 M6 M7 M8 M9 solution Temperature for thermal 200 200 200 200 200 200200 200 treatment (° C.) Grid voltage (V) −600 −600 −600 −600 −600 −600−600 −600 Initial surface voltage (V) −1,324 −1,401 −1,178 −939 −1,168−1,183 −1,094 −1,314 Storage time (hr) 350 650 210 70 700 550 260 550Surface voltage before −1,149 −1,025 −1,101 −927 −1,163 −1,170 −1,071−1,117 TSD heating (V) Discharge peak 189 192 170 203 182 189 180 172temperature (° C.) Discharge initiation 147 151 133 158 147 155 139 140temperature (° C.)

When the discharge initiation temperatures shown in Table 6 arecompared, it is evident that the discharge initiation temperature ineach of Examples wherein the temperature for the thermal treatment wasfrom 250 to 330° C., exceeded the discharge initiation temperature inComparative Examples wherein the discharge initiation temperatures wasless than 250° C.

Further, when the discharge initiation temperatures shown in Tables 7and 8 are compared between Examples wherein the same polymer compositionsolution was used, it is evident that the discharge initiationtemperature in each of Examples wherein the temperature for the thermaltreatment was from 260 to 280° C., exceeded the discharge initiationtemperatures in Comparative Examples wherein the temperature for thethermal treatment was 200° C.

Test Example 4

[Measurement of IR Spectrum]

On a polytetrafluoroethylene sheet (hereinafter referred to as a “PTFEsheet”), the polymer composition solution M1 was cast to form a film,which was subjected to pretreatment at 100° C. for one hour and thenthermally treated for one hour at the heat treatment temperaturedisclosed in Table 7 and peeled from the PTFE sheet to obtain a castfilm having a thickness of from 50 to 100 μm.

The infrared absorption spectrum of each cast film was measured by meansof AVATAR370FT-IR manufactured by Thermo Nicolet, whereupon the areas ofthe absorption at 1,670 cm⁻¹ (hereinafter referred to as peak (α))derived from the amino group of γ-aminopropylmethyldiethoxysilane (N—Hbending vibration), the absorption at 1,730 cm⁻¹ (hereinafter referredto as peak (β)) derived from the carbonyl group of an amide group formedby a reaction of γ-aminopropylmethyldiethoxysilane with the terminalgroup COOH group of polyperfluorobutenyl vinyl ether A2, and theabsorption at from 2,800 cm⁻¹ to 3,000 cm⁻¹ (hereinafter referred to aspeak (y)) derived from the C—H bond of the ethoxy group ofγ-aminopropylmethyldiethoxysilane, were quantified.

Peak area-standardized values obtained by standardizing the areas ofpeaks (α), (β) and (γ) by the area of the absorption at from 2,000 cm⁻¹to 2,700 cm⁻¹ (hereinafter referred to as peak (δ)) derived from CF₂ ofpolyperfluorobutenyl vinyl ether A2 by the following formula, are shownin Table 9 and FIG. 4.(Peak area-standardized value)=(area of either peak (α), (β) or(γ))/(area of peak (δ))×100

TABLE 9 Test Test Test Test Test Test Test Test Ex. 7 Ex. 6 Ex. 5 Ex. 4Ex. 3 Ex. 2 Ex. 1 Ex. 8 Polymer M1 M1 M1 M1 M1 M1 M1 M1 compositionsolution Temperature 200 230 240 250 260 280 300 330 for thermaltreatment (° C.) Peak area- 5.6 1.3 1.2 1.0 0.7 0.2 0.2 0.02standardized value (α) Peak area- 1.3 2.0 2.6 3.0 3.4 3.1 3.3 1.8standardized value (β) Peak area- 9.5 6.5 6.3 5.6 4.5 2.8 2.4 0.7standardized value (γ)

As shown in Table 9 and FIG. 4, the peak area-standardized value of peak(α) derived from the amino group of γ-aminopropylmethyldiethoxysilanedecreased as the temperature became high, and it became a value close tosubstantially zero at a temperature of at least 280° C.

It was thereby found that in order to promote the reaction ofγ-aminopropylmethyldiethoxysilane with the terminal group COOH group ofpolyperfluorobutenyl vinyl ether A2, the temperature for the thermaltreatment should better be high, but should be sufficient if it is atleast 280° C.

On the other hand, the peak area-standardized value of peak (β) derivedfrom the carbonyl group of the amide group formed by a reaction ofγ-aminopropylmethyldiethoxysilane with the terminal group COOH group ofpolyperfluorobutenyl vinyl ether A2, increases as the temperature rises,i.e. as the peak area-standardized value of peak (α) decreases, within arange of from 250° C. to 260° C. However, at a temperature of at least250° C. to 260° C., it becomes substantially constant, and when thetemperature exceeds 300° C., it decreases.

It was thereby found that the bonding by the reaction ofγ-aminopropylmethyldiethoxysilane with the terminal group COOH group ofpolyperfluorobutenyl vinyl ether A2 becomes sufficient at a temperatureof at least 250° C. to 260° C. and that although it turns to decreasewhen the temperature exceeds 300° C., high level bonding is stillmaintained at 330° C.

Further, the peak area-standardized value of peak (γ) derived from theC—H bond of the ethoxy group of γ-aminopropylmethyldiethoxysilane,decreased as the temperature rose and became 0.7 at 330° C.

It was thereby found that the condensation reaction ofγ-aminopropylmethyldiethoxysilane is promoted as the temperature ishigh.

From the foregoing results, it was confirmed that the temperature forthe thermal treatment at which preferred results were obtained in TestExamples 1 to 3, corresponds to the temperature for the thermaltreatment at which bonding of the polymer (A) and the silane couplingagent is promoted and maintained, and at which the condensation reactionof a silane coupling agent is promoted.

Test Example 5

[Small Angle X-Ray Scattering Analysis]

On a PTFE sheet, the polymer composition solution M1 was cast to form afilm, which was dried under conditions of 100° C. for one hour and 280°C. for one hour to prepare a coating film A having a film thickness ofabout 100 μm. Further, film formation was carried out in the samemanner, and the drying conditions were changed to 100° C. for one hourand 200° C. for one hour to prepare a coating film B. Further, using thepolymer solution P1, film formation was carried out in the same manneras for the coating film B to prepare a coating film X. Then, asmall-angle X-ray scattering measurement was carried out by using theabove-mentioned coating films A, B and X by Nano-viewer manufactured byRigaku Corporation. The measurement conditions were as shown below.

X-ray wavelength: 0.154 nm (CuKα ray)

Length of camera: 500 mm

Detector: IP (imaging plate)

Measurement mode: transmission measurement

Measurement temperature: room temperature

Exposure time: 30 minutes

Optical type slit; 1st 0.4 mm, 2nd 0.3 mm, 3rd 0.5 mm

The result of a small-angle X-ray ray scattering measurement of a castfilm was shown in FIG. 5. In FIG. 5, the vertical axis represents theintensity (optional unit) of X-ray scattering, and q of the horizontalaxis represents a value of the following formula. In the followingformula, λ is a wavelength, and θ is a scattering angle.q=4π/λ×sin(θ/2)

In FIG. 5, A, B and X respectively represent scattering spectra of thecoating films A, B and X.

From this result, in the case of the coating films A and B employing thepolymer composition solution M1 having γ-aminopropylmethyldiethoxysilanemixed therein, a scattering peak which was not observed in the coatingfilm X employing the polymer solution P1 having no such silane mixedtherein, was detected. This indicates that in a uniform film in a statewhere no γ-aminopropylmethyldiethoxysilane is present, a non-uniformstructure derived from γ-aminopropylmethyldiethoxysilane was formed. Thesize (D) of the non-uniform structure can be calculated from the value q(see FIG. 5) which can be read from the peak top of the scattering peakof this result (D=2π/q). Table 10 shows the temperature for the thermaltreatment at the time of preparing the coating films A, B and X and thesizes (D) of the obtained non-uniform structures. From such results, itis estimated that a non-uniform portion of about 28 nm is present in thecoating film A, and a non-uniform portion of about 20 nm is present inthe coating film B.

From the results in FIG. 5 and Table 10, it is estimated that in thefluorinated polymer, γ-aminopropylmethyldiethoxysilane and itscondensate form a nano cluster, and the size of this nano cluster tendsto increase as the temperature for the thermal treatment becomes high.It is considered that this nano cluster works as an electric chargeretaining portion of the electret and thus contributes to improvement ofthe above-described stability with time and thermal stability ofretained electric charge, and it is considered that the improvement inthe thermal stability of the electret in the present invention isbrought about by an increase in the size of such a nano cluster.

TABLE 10 Cast film A B X Polymer (composition) solution M1 M1 P1Temperature for thermal treatment (° C.) 280 200 200 Value q 0.22 0.31Nil Size of non-uniform structure (nm) 28 20 —

Test Example 6 Examples 41 to 44 and Comparative Examples 41 to 44

(1) Preparation of Electrets

An electret in each Example or Comparative Example was prepared in thesame manner as in Example 8 except that the polymer composition solutionwas changed to the polymer composition solution in Tables 11 and 12, andthe temperature for the thermal treatment and the grid voltage wereadjusted to the values in Tables 11 and 12.

(2) Measurement of Surface Voltages

With respect to each electret thus prepared, as intended for anelectrostatic induction conversion device for vehicles, the followingrespective surface voltages were measured, and the surface voltageresidual ratio was obtained in the same manner as in Example 8. Theresults are shown in Tables 11 and 12.

TABLE 11 Ex. 41 Ex. 42 Ex. 43 Ex. 44 Polymer composition solution M10M11 M12 M13 Temperature for thermal 260 280 280 280 treatment (° C.)Grid voltage (V) −1,200 −1,200 −1,200 −1,200 Initial surface voltage (V)−1,102 −1,087 −1,082 −1,067 Storage time (hr) 165 720 165 300 Surfacevoltage −1,095 −1,010 −1,060 −983 before heating at 125° C. (V) Surfacevoltage −929 −853 −984 −876 after heating at 125° C. (V) Surface voltage85 85 91 89 residual ratio (%)

TABLE 12 Comp. Comp. Comp. Comp. Ex. 41 Ex. 42 Ex. 43 Ex. 44 Polymercomposition solution M10 M11 M12 M13 Temperature for thermal 200 200 200200 treatment (° C.) Grid voltage (V) −1,200 −1,200 −1,200 −1,200Initial surface voltage (V) −1,090 −1,065 −1,104 −1,070 Storage time(hr) 165 300 165 300 Surface voltage −1,080 −1,050 −1,070 −1,000 beforeheating at 125° C. (V) Surface voltage −594 −556 −696 −640 after heatingat 125° C. (V) Surface voltage 55 53 65 64 residual ratio (%)

When the surface voltage residual ratios shown in Tables 11 and 12 arecompared with respect to cases whereinN-(β-aminoethyl)-γ-aminopropylmethyldimethoxysilane orm-aminophenyltrimethoxysilane is used as a silane coupling agent, it isevident that when Examples wherein the same polymer composition solutionwas used, are compared, the surface voltage residual ratio in each ofExamples wherein the temperature for the thermal treatment was from 260to 280° C., exceeded the surface voltage residual ratios in ComparativeExamples wherein the temperature for the thermal treatment was 200° C.

Test Example 7 Examples 51 to 56 and Comparative Examples 51 to 55

(1) Preparation of Electrets

An electret in each Example or Comparative Example was prepared in thesame manner as in Example 20 except that the polymer compositionsolution was changed to the polymer composition solution in Tables 13and 14, and the temperature for the thermal treatment and the gridvoltage were adjusted to the values in Tables 13 and 14.

(2) Measurement of Surface Voltages

With respect to each electret thus prepared, the respective surfacevoltages were measured in the same manner as in Example 20. The resultsare shown in Tables 13 and 14.

TABLE 13 Comp. Comp. Ex. 51 Ex. 52 Ex. 53 Ex. 51 Ex. 52 Polymercomposition M12 M12 M12 M12 M12 solution Temperature for thermal 300 280260 230 200 treatment (° C.) Grid voltage (V) −1,200 −1,200 −1,200−1,200 −1,200 Initial surface voltage −1,191 −1,091 −1,080 −1,105 −1,075(V) Storage time (hr) 290 165 140 120 48 Surface voltage before −1,079−1,056 −961 −1,091 −1,050 TSD heating (V) Discharge peak 258 249 228 205188 temperature (° C.) Discharge initiation 174 177 174 170 149temperature (° C.)

TABLE 14 Comp. Comp. Comp. Ex. 54 Ex. 55 Ex. 56 Ex. 53 Ex. 54 Ex. 55Polymer M10 M12 M13 M10 M11 M13 composition solution Temperature 260 280280 200 200 200 for thermal treatment (° C.) Grid −1,200 −1,200 −1,200−1,100 −600 −1,200 voltage (V) Initial surface −1,042 −1,093 −1,128−1,084 −1,280 −1,080 voltage (V) Storage 360 165 140 340 190 280 time(hr) Surface voltage −1,000 −992 −1,095 −1,054 −1,202 −1,036 before TSDheating (V) Discharge 223 226 224 164 189 195 peak tem- perature (° C.)Discharge 163 161 165 136 129 137 initiation tem- perature (° C.)

When the discharge initiation temperatures shown in Table 13 arecompared with respect to cases whereinN-(β-aminoethyl)-γ-aminopropylmethyldimethoxysilane orm-aminophenyltrimethoxysilane was used as a silane coupling agent, it isevident that the discharge initiation temperature in each of Exampleswherein the temperature for the thermal treatment was from 260 to 300°C., exceeded the discharge initiation temperatures in ComparativeExamples wherein the discharge initiation temperature was less than 250°C.

Further, when the discharge initiation temperatures shown in Table 14are compared between Examples wherein the same polymer compositionsolution was used, it is evident that the discharge initiationtemperature in each of Examples wherein the temperature for the thermaltreatment was from 260 to 280° C., exceeded the discharge initiationtemperature in Comparative Examples wherein the temperature for thethermal treatment was 200° C.

Industrial Applicability

The present invention is useful for the production of an electret whichis used for an electrostatic induction conversion device such as apower-generating unit, a microphone, etc.

The entire disclosure of Japanese Patent Application No. 2009-038508filed on Feb. 20, 2009 including specification, claims, drawings andsummary is incorporated herein by reference in its entirety.

Reference Symbols

10: copper substrate, 11: coating film, 12: DC high-voltage powersource, 14: corona needle, 16: grid, 17: ammeter, 18: power source forgrid, 19: hotplate, 20: counter electrode, 21: electret, 22: ammeter.

1. A process for producing an electret, which comprises a step ofthermally treating a composition comprising a fluorinated polymer and asilane coupling agent, wherein the fluorinated polymer has an alicyclicstructure in its main chain and has a carboxy group or an alkoxycarbonylgroup as its terminal group; the silane coupling agent has an aminogroup; and the temperature for the thermal treatment is from 250° C. to330° C.
 2. The process for producing an electret according to claim 1,wherein the fluorinated polymer has, as the alicyclic structure, afluorinated alicyclic structure in its main chain.
 3. The process forproducing an electret according to claim 1, wherein the fluorinatedpolymer has, as the alicyclic structure, a cyclic structure containingan etheric oxygen atom, in its main chain.
 4. The process for producingan electret according to claim 1, wherein the fluorinated polymer has,as the alicyclic structure, a fluorinated alicyclic structure containingan etheric oxygen atom, in its main chain.
 5. The process for producingan electret according to claim 1, wherein the weight average molecularweight of the fluorinated polymer is from 150,000 to 650,000.
 6. Theprocess for producing an electret according to claim 1, wherein thesilane coupling agent is at least one member selected from the groupconsisting of γ-aminopropylmethyldiethoxysilane,γ-aminopropylmethyldimethoxysilane,N-(β-aminoethyl)-γ-aminopropylmethyldimethoxysilane,N-(β-aminoethyl)-γ-aminopropylmethyldiethoxysilane,γ-aminopropyltrimethoxysilane, γ-aminopropyltriethoxysilane,N-(β-aminoethyl)-γ-aminopropyltrimethoxysilane,N-(β-aminoethyl)-γ-aminopropyltriethoxysilane andaminophenyltrimethoxysilane.
 7. The process for producing an electretaccording to claim 1, wherein the content of the silane coupling agentis from 0.1 to 20 mass %, based on the total amount of the fluorinatedpolymer and the silane coupling agent.
 8. The process for producing anelectret according to claim 1, which includes the following (1) to (4)in the order of (1), (2), (3) and (4): (1) a step of obtaining a coatingfluid having the fluorinated polymer and the silane coupling agentdissolved in a solvent (a coating fluid-preparation step), (2) a step ofcoating a substrate with the coating fluid to form a coating layercomprising the fluorinated polymer and the silane coupling agent (acoating step), (3) a step of thermally treating the coating layer toobtain a coating film (a thermal treatment step), and (4) a step ofinjecting an electric charge to the coating film after the thermaltreatment.
 9. The process for producing an electret according to claim8, wherein the solvent is a fluorinated organic solvent.
 10. Anelectrostatic induction conversion device comprising the electretobtained by the process as defined in claim 1.